formats/config: Added encoder/decoder for git config files. (#97)

* WIP: Add config format parser.

* add decoder based on gcfg.

Portions of code taken from:
https://github.com/go-gcfg/gcfg/blob/5b9f94ee80b2331c3982477bd84be8edd857df33/read.go

* add git config encoder and config methods.

* use format/config in storage/filesystem for read

* use format/config in storage/filesystem to write

* formats/config: improve docs.

* formats/config: improve tests.

* formats/config: use our fork of gcfg; improve api.

* formats/config: improve api.

* storage/filesystem: fix gofmt

* formats/config: use NoSubsection constant.

* formats/config: add doc.go

* formats/config: requested sytle changes.

* formats/config: do not use *_test packages.

1 files changed, 200 insertions, 0 deletions

diff --git a/formats/config/doc.go b/formats/config/doc.go
new file mode 100644
index 0000000..1f7eb78
--- /dev/null
+++ b/formats/config/doc.go
@@ -0,0 +1,200 @@
+// Package config implements decoding, encoding and manipulation
+// of git config files.
+package config
+
+/*
+
+CONFIGURATION FILE
+------------------
+
+The Git configuration file contains a number of variables that affect
+the Git commands' behavior. The `.git/config` file in each repository
+is used to store the configuration for that repository, and
+`$HOME/.gitconfig` is used to store a per-user configuration as
+fallback values for the `.git/config` file. The file `/etc/gitconfig`
+can be used to store a system-wide default configuration.
+
+The configuration variables are used by both the Git plumbing
+and the porcelains. The variables are divided into sections, wherein
+the fully qualified variable name of the variable itself is the last
+dot-separated segment and the section name is everything before the last
+dot. The variable names are case-insensitive, allow only alphanumeric
+characters and `-`, and must start with an alphabetic character.  Some
+variables may appear multiple times; we say then that the variable is
+multivalued.
+
+Syntax
+~~~~~~
+
+The syntax is fairly flexible and permissive; whitespaces are mostly
+ignored.  The '#' and ';' characters begin comments to the end of line,
+blank lines are ignored.
+
+The file consists of sections and variables.  A section begins with
+the name of the section in square brackets and continues until the next
+section begins.  Section names are case-insensitive.  Only alphanumeric
+characters, `-` and `.` are allowed in section names.  Each variable
+must belong to some section, which means that there must be a section
+header before the first setting of a variable.
+
+Sections can be further divided into subsections.  To begin a subsection
+put its name in double quotes, separated by space from the section name,
+in the section header, like in the example below:
+
+--------
+	[section "subsection"]
+
+--------
+
+Subsection names are case sensitive and can contain any characters except
+newline (doublequote `"` and backslash can be included by escaping them
+as `\"` and `\\`, respectively).  Section headers cannot span multiple
+lines.  Variables may belong directly to a section or to a given subsection.
+You can have `[section]` if you have `[section "subsection"]`, but you
+don't need to.
+
+There is also a deprecated `[section.subsection]` syntax. With this
+syntax, the subsection name is converted to lower-case and is also
+compared case sensitively. These subsection names follow the same
+restrictions as section names.
+
+All the other lines (and the remainder of the line after the section
+header) are recognized as setting variables, in the form
+'name = value' (or just 'name', which is a short-hand to say that
+the variable is the boolean "true").
+The variable names are case-insensitive, allow only alphanumeric characters
+and `-`, and must start with an alphabetic character.
+
+A line that defines a value can be continued to the next line by
+ending it with a `\`; the backquote and the end-of-line are
+stripped.  Leading whitespaces after 'name =', the remainder of the
+line after the first comment character '#' or ';', and trailing
+whitespaces of the line are discarded unless they are enclosed in
+double quotes.  Internal whitespaces within the value are retained
+verbatim.
+
+Inside double quotes, double quote `"` and backslash `\` characters
+must be escaped: use `\"` for `"` and `\\` for `\`.
+
+The following escape sequences (beside `\"` and `\\`) are recognized:
+`\n` for newline character (NL), `\t` for horizontal tabulation (HT, TAB)
+and `\b` for backspace (BS).  Other char escape sequences (including octal
+escape sequences) are invalid.
+
+
+Includes
+~~~~~~~~
+
+You can include one config file from another by setting the special
+`include.path` variable to the name of the file to be included. The
+variable takes a pathname as its value, and is subject to tilde
+expansion.
+
+The
+included file is expanded immediately, as if its contents had been
+found at the location of the include directive. If the value of the
+`include.path` variable is a relative path, the path is considered to be
+relative to the configuration file in which the include directive was
+found.  See below for examples.
+
+
+Example
+~~~~~~~
+
+	# Core variables
+	[core]
+		; Don't trust file modes
+		filemode = false
+
+	# Our diff algorithm
+	[diff]
+		external = /usr/local/bin/diff-wrapper
+		renames = true
+
+	[branch "devel"]
+		remote = origin
+		merge = refs/heads/devel
+
+	# Proxy settings
+	[core]
+		gitProxy="ssh" for "kernel.org"
+		gitProxy=default-proxy ; for the rest
+
+	[include]
+		path = /path/to/foo.inc ; include by absolute path
+		path = foo ; expand "foo" relative to the current file
+		path = ~/foo ; expand "foo" in your `$HOME` directory
+
+
+Values
+~~~~~~
+
+Values of many variables are treated as a simple string, but there
+are variables that take values of specific types and there are rules
+as to how to spell them.
+
+boolean::
+
+       When a variable is said to take a boolean value, many
+       synonyms are accepted for 'true' and 'false'; these are all
+       case-insensitive.
+
+       true;; Boolean true can be spelled as `yes`, `on`, `true`,
+		or `1`.  Also, a variable defined without `= <value>`
+		is taken as true.
+
+       false;; Boolean false can be spelled as `no`, `off`,
+		`false`, or `0`.
++
+When converting value to the canonical form using `--bool` type
+specifier; 'git config' will ensure that the output is "true" or
+"false" (spelled in lowercase).
+
+integer::
+       The value for many variables that specify various sizes can
+       be suffixed with `k`, `M`,... to mean "scale the number by
+       1024", "by 1024x1024", etc.
+
+color::
+       The value for a variable that takes a color is a list of
+       colors (at most two, one for foreground and one for background)
+       and attributes (as many as you want), separated by spaces.
++
+The basic colors accepted are `normal`, `black`, `red`, `green`, `yellow`,
+`blue`, `magenta`, `cyan` and `white`.  The first color given is the
+foreground; the second is the background.
++
+Colors may also be given as numbers between 0 and 255; these use ANSI
+256-color mode (but note that not all terminals may support this).  If
+your terminal supports it, you may also specify 24-bit RGB values as
+hex, like `#ff0ab3`.
++
+
+From: https://git-scm.com/docs/git-config
+The accepted attributes are `bold`, `dim`, `ul`, `blink`, `reverse`,
+`italic`, and `strike` (for crossed-out or "strikethrough" letters).
+The position of any attributes with respect to the colors
+(before, after, or in between), doesn't matter. Specific attributes may
+be turned off by prefixing them with `no` or `no-` (e.g., `noreverse`,
+`no-ul`, etc).
++
+For git's pre-defined color slots, the attributes are meant to be reset
+at the beginning of each item in the colored output. So setting
+`color.decorate.branch` to `black` will paint that branch name in a
+plain `black`, even if the previous thing on the same output line (e.g.
+opening parenthesis before the list of branch names in `log --decorate`
+output) is set to be painted with `bold` or some other attribute.
+However, custom log formats may do more complicated and layered
+coloring, and the negated forms may be useful there.
+
+pathname::
+	A variable that takes a pathname value can be given a
+	string that begins with "`~/`" or "`~user/`", and the usual
+	tilde expansion happens to such a string: `~/`
+	is expanded to the value of `$HOME`, and `~user/` to the
+	specified user's home directory.
+
+From:
+https://raw.githubusercontent.com/git/git/659889482ac63411daea38b2c3d127842ea04e4d/Documentation/config.txt
+
+*/