move plumbing from top level package to plumbing (#183)

* plumbing: rename Object -> EncodedObject.

* plumbing/storer: rename ObjectStorer -> EncodedObjectStorer.

* move difftree to plumbing/difftree.

* move diff -> utils/diff

* make Object/Tag/Blob/Tree/Commit/File depend on storer.

* Object and its implementations now depend only on
  storer.EncodedObjectStorer, not git.Repository.
* Tests are decoupled accordingly.

* move Object/Commit/File/Tag/Tree to plumbing/object.

* move Object/Commit/File/Tag/Tree to plumbing/object.
* move checkClose to utils/ioutil.

* move RevListObjects to plumbing/revlist.Objects.

* move DiffTree to plumbing/difftree package.

* rename files with plural nouns to singular

* plumbing/object: add GetBlob/GetCommit/GetTag/GetTree.

1 files changed, 30 insertions, 0 deletions

diff --git a/plumbing/difftree/internal/merkletrie/doc.go b/plumbing/difftree/internal/merkletrie/doc.go
new file mode 100644
index 0000000..f8c3b2f
--- /dev/null
+++ b/plumbing/difftree/internal/merkletrie/doc.go
@@ -0,0 +1,30 @@
+package merkletrie
+
+/*
+Package merkletrie gives support for n-ary trees that are at the same
+time Merkle trees and Radix trees, and provides an efficient tree
+comparison algorithm for them.
+
+Git trees are Radix n-ary trees in virtue of the names of their
+tree entries.  At the same time, git trees are Merkle trees thanks to
+their hashes.
+
+When comparing git trees, the simple approach of alphabetically sorting
+their elements and comparing the resulting lists is not enough as it
+depends linearly on the number of files in the trees: When a directory
+has lots of files but none of them has been modified, this approach is
+very expensive.  We can do better by prunning whole directories that
+have not change, by just by looking at their hashes.  This package
+provides the tools to do exactly that.
+
+This package defines Radix-Merkle trees as nodes that should have:
+- a hash: the Merkle part of the Radix-Merkle tree
+- a key: the Radix part of the Radix-Merkle tree
+
+The Merkle hash condition is not enforced by this package though.  This
+means that node hashes doesn't have to take into account the hashes of
+their children,  which is good for testing purposes.
+
+Nodes in the Radix-Merkle tree are abstracted by the Noder interface.
+The intended use is that git.Tree implements this interface.
+*/