Allow parsing very long lines in commit messages

It's technically possible for commit messages to contain arbitrary
non-NUL bytes.  Our commit message parser uses a 64 KiB buffer to read
lines, and if lines are too long, it will fail.  We've seen some cases
where commit message lines are much longer due to historical accidents.

Let's raise the maximum buffer size to 1 MiB to accommodate these
situations.  Let's also make it easier to track problems by returning a
more helpful error message if the buffer size is exceeded.

Author: bk2204

commit.go

@@ -96,6 +96,7 @@ func (c *Commit) Decode(from io.Reader, size int64) (n int, err error) {
 	var messageParts []string
 
 	s := bufio.NewScanner(from)
+	s.Buffer(nil, 1024 * 1024)
 	for s.Scan() {
 		text := s.Text()
 		n = n + len(text+"\n")
@@ -169,7 +170,7 @@ func (c *Commit) Decode(from io.Reader, size int64) (n int, err error) {
 	c.Message = strings.Join(messageParts, "\n")
 
 	if err = s.Err(); err != nil {
-		return n, err
+		return n, fmt.Errorf("failed to parse commit buffer: %s", err)
 	}
 	return n, err
 }

commit_test.go

@@ -117,6 +117,37 @@ func TestCommitDecodingWithEmptyName(t *testing.T) {
 	assert.Equal(t, "initial commit", commit.Message)
 }
 
+func TestCommitDecodingWithLargeCommitMessage(t *testing.T) {
+	message := "This message text is, with newline, exactly 64 characters long. ";
+	// This message will be exactly 1 MiB in size when part of the commit.
+	longMessage := strings.Repeat(message, (1024 * 1024 / 64) - 1)
+	longMessage += strings.TrimSpace(message);
+
+	author := &Signature{Name: "", Email: "john@example.com", When: time.Now()}
+	committer := &Signature{Name: "", Email: "jane@example.com", When: time.Now()}
+
+	treeId := []byte("cccccccccccccccccccc")
+
+	from := new(bytes.Buffer)
+
+	fmt.Fprintf(from, "author %s\n", author)
+	fmt.Fprintf(from, "committer %s\n", committer)
+	fmt.Fprintf(from, "tree %s\n", hex.EncodeToString(treeId))
+	fmt.Fprintf(from, "\n%s\n", longMessage)
+
+	flen := from.Len()
+
+	commit := new(Commit)
+	n, err := commit.Decode(from, int64(flen))
+
+	assert.Nil(t, err)
+	assert.Equal(t, flen, n)
+
+	assert.Equal(t, author.String(), commit.Author)
+	assert.Equal(t, committer.String(), commit.Committer)
+	assert.Equal(t, longMessage, commit.Message)
+}
+
 func TestCommitDecodingWithMessageKeywordPrefix(t *testing.T) {
 	author := &Signature{Name: "John Doe", Email: "john@example.com", When: time.Now()}
 	committer := &Signature{Name: "Jane Doe", Email: "jane@example.com", When: time.Now()}