error strings: Don't capitalize, use periods, or newlines

Per [Go Code Review Comments][1],

> Error strings should not be capitalized (unless beginning with proper
> nouns or acronyms) or end with punctuation

staticcheck's [ST1005][2] also complains about these. For example,

```
object_walker.go:63:10: error strings should not be capitalized (ST1005)
object_walker.go:101:10: error strings should not be capitalized (ST1005)
object_walker.go:101:10: error strings should not end with punctuation or a newline (ST1005)
plumbing/format/commitgraph/file.go:17:26: error strings should not be capitalized (ST1005)
```

This fixes all instances of this issue reported by staticcheck.

  [1]: https://github.com/golang/go/wiki/CodeReviewComments#error-strings
  [2]: https://staticcheck.io/docs/checks/#ST1005

3 files changed, 6 insertions, 6 deletions

diff --git a/plumbing/format/commitgraph/file.go b/plumbing/format/commitgraph/file.go
index 0ce7198..1d25238 100644
--- a/plumbing/format/commitgraph/file.go
+++ b/plumbing/format/commitgraph/file.go
@@ -14,14 +14,14 @@ import (
 var (

 	// ErrUnsupportedVersion is returned by OpenFileIndex when the commit graph

 	// file version is not supported.

-	ErrUnsupportedVersion = errors.New("Unsupported version")

+	ErrUnsupportedVersion = errors.New("unsupported version")

 	// ErrUnsupportedHash is returned by OpenFileIndex when the commit graph

 	// hash function is not supported. Currently only SHA-1 is defined and

 	// supported

-	ErrUnsupportedHash = errors.New("Unsupported hash algorithm")

+	ErrUnsupportedHash = errors.New("unsupported hash algorithm")

 	// ErrMalformedCommitGraphFile is returned by OpenFileIndex when the commit

 	// graph file is corrupted.

-	ErrMalformedCommitGraphFile = errors.New("Malformed commit graph file")

+	ErrMalformedCommitGraphFile = errors.New("malformed commit graph file")

 

 	commitFileSignature    = []byte{'C', 'G', 'P', 'H'}

 	oidFanoutSignature     = []byte{'O', 'I', 'D', 'F'}

diff --git a/plumbing/format/gitattributes/attributes.go b/plumbing/format/gitattributes/attributes.go
index d13c2a9..329e667 100644
--- a/plumbing/format/gitattributes/attributes.go
+++ b/plumbing/format/gitattributes/attributes.go
@@ -15,7 +15,7 @@ const (
 
 var (
 	ErrMacroNotAllowed      = errors.New("macro not allowed")
-	ErrInvalidAttributeName = errors.New("Invalid attribute name")
+	ErrInvalidAttributeName = errors.New("invalid attribute name")
 )
 
 type MatchAttribute struct {
diff --git a/plumbing/format/idxfile/decoder.go b/plumbing/format/idxfile/decoder.go
index 7768bd6..51a3904 100644
--- a/plumbing/format/idxfile/decoder.go
+++ b/plumbing/format/idxfile/decoder.go
@@ -12,9 +12,9 @@ import (
 var (
 	// ErrUnsupportedVersion is returned by Decode when the idx file version
 	// is not supported.
-	ErrUnsupportedVersion = errors.New("Unsupported version")
+	ErrUnsupportedVersion = errors.New("unsupported version")
 	// ErrMalformedIdxFile is returned by Decode when the idx file is corrupted.
-	ErrMalformedIdxFile = errors.New("Malformed IDX file")
+	ErrMalformedIdxFile = errors.New("malformed IDX file")
 )
 
 const (