commit: fix parsing of mergetag headers

Git contains some multiline headers, including `gpgsig`,
`gpgsig-sha256`, and `mergetag` headers.  These headers start with a
normal line of data and then are continued after a newline by
indentation with a single space, which is stripped when computing the
body.

Our header parsing for commits could misparse these because we use
`strings.Fields`, which strips leading space.  As a result, if the
mergetag message contains a line starting with "tree " and then a
non-hex character, we'd fail parsing since we'd attempt to parse it as a
tree header.  Let's fix this by using `strings.Split` instead, which
doesn't strip space and therefore won't try to misparse this.

The fake commit is synthesized from
be122abe4bcd6d39b37892daae28c8bf5e4030fc in the Linux kernel repository.
Identifying information and most of the text are removed for privacy and
license reasons.

Author: bk2204

commit.go

@@ -107,7 +107,7 @@ func (c *Commit) Decode(hash hash.Hash, from io.Reader, size int64) (n int, err
 			continue
 		}
 
-		if fields := strings.Fields(text); !finishedHeaders {
+		if fields := strings.Split(text, " "); !finishedHeaders {
 			if len(fields) == 0 {
 				// Executing in this block means that we got a
 				// whitespace-only line, while parsing a header.

commit_test.go

@@ -238,6 +238,65 @@ func TestCommitDecodingMultilineHeader(t *testing.T) {
 		strings.Split(hdr.V, "\n"))
 }
 
+func TestCommitDecodingMessageWithLineStartingWithTree(t *testing.T) {
+	from := new(bytes.Buffer)
+
+	// The tricky part here that we're testing is the "tree support" in the
+	// `mergetag` header, which we should not try to parse as a tree header.
+	// Note also that this entry contains trailing whitespace which must not
+	// be trimmed.
+	fmt.Fprintf(from, `tree e8ad84c41c2acde27c77fa212b8865cd3acfe6fb
+parent b343c8beec664ef6f0e9964d3001c7c7966331ae
+parent 1e8a52e18cfb381bc9cc1f0b720540364d2a6edd
+author Pat Doe <[email protected]> 1337892984 -0700
+committer Pat Doe <[email protected]> 1337892984 -0700
+mergetag object 1e8a52e18cfb381bc9cc1f0b720540364d2a6edd
+ type commit
+ tag random
+ tagger J. Roe <[email protected]> 1337889148 -0600
+ 
+ Random changes
+ 
+ This text contains some
+ tree support code.
+ -----BEGIN PGP SIGNATURE-----
+ Version: GnuPG v1.4.11 (GNU/Linux)
+ 
+ Not a real signature
+ -----END PGP SIGNATURE-----
+
+Merge tag 'random' of git://git.example.ca/git/
+`)
+
+	flen := from.Len()
+
+	commit := new(Commit)
+	n, err := commit.Decode(sha1.New(), from, int64(flen))
+
+	require.Nil(t, err)
+	require.Equal(t, flen, n)
+	require.Equal(t, commit.ExtraHeaders, []*ExtraHeader{
+		{
+			K: "mergetag",
+			V: `object 1e8a52e18cfb381bc9cc1f0b720540364d2a6edd
+type commit
+tag random
+tagger J. Roe <[email protected]> 1337889148 -0600
+
+Random changes
+
+This text contains some
+tree support code.
+-----BEGIN PGP SIGNATURE-----
+Version: GnuPG v1.4.11 (GNU/Linux)
+
+Not a real signature
+-----END PGP SIGNATURE-----`},
+	},
+	)
+	require.Equal(t, commit.Message, "Merge tag 'random' of git://git.example.ca/git/")
+}
+
 func assertLine(t *testing.T, buf *bytes.Buffer, wanted string, args ...interface{}) {
 	got, err := buf.ReadString('\n')
 	if err == io.EOF {