Add description parsing to allow subtree merges

Now git commit messages are parsed so that their contents can be
examined. This is similar to the commits that are exempt from validation using
the bump prefix.

tools/rebasehelpers/commitchecker/commitchecker.go

@@ -27,9 +27,11 @@ func main() {
 
 	// TODO: Filter out bump commits for now until we decide how to deal with
 	// them correctly.
+	// TODO: ...along with subtree merges.
 	nonbumpCommits := []util.Commit{}
 	for _, commit := range commits {
-		if !strings.HasPrefix(commit.Summary, "bump(") {
+		lastDescriptionLine := commit.Description[len(commit.Description)-1]
+		if !strings.HasPrefix(commit.Summary, "bump(") && !strings.HasPrefix(lastDescriptionLine, "git-subtree-split:") {
 			nonbumpCommits = append(nonbumpCommits, commit)
 		}
 	}

tools/rebasehelpers/util/git.go

@@ -31,9 +31,10 @@ var SupportedHosts = map[string]int{
 }
 
 type Commit struct {
-	Sha     string
-	Summary string
-	Files   []File
+	Sha         string
+	Summary     string
+	Description []string
+	Files       []File
 }
 
 func (c Commit) DeclaresUpstreamChange() bool {
@@ -214,12 +215,14 @@ func CommitsBetween(a, b string) ([]Commit, error) {
 
 func NewCommitFromOnelineLog(log string) (Commit, error) {
 	var commit Commit
+	var err error
 	parts := strings.Split(log, " ")
 	if len(parts) < 2 {
 		return commit, fmt.Errorf("invalid log entry: %s", log)
 	}
 	commit.Sha = parts[0]
 	commit.Summary = strings.Join(parts[1:], " ")
+	commit.Description, err = descriptionInCommit(commit.Sha)
 	files, err := filesInCommit(commit.Sha)
 	if err != nil {
 		return commit, err
@@ -335,6 +338,22 @@ func filesInCommit(sha string) ([]File, error) {
 	return files, nil
 }
 
+func descriptionInCommit(sha string) ([]string, error) {
+	descriptionLines := []string{}
+	stdout, stderr, err := run("git", "show", "--quiet", sha)
+	if err != nil {
+		return descriptionLines, fmt.Errorf("%s: %s", stderr, err)
+	}
+
+	for _, commitLine := range strings.Split(stdout, "\n") {
+		if len(commitLine) == 0 {
+			continue
+		}
+		descriptionLines = append(descriptionLines, strings.Trim(commitLine, " "))
+	}
+	return descriptionLines, nil
+}
+
 func run(args ...string) (string, string, error) {
 	cmd := exec.Command(args[0], args[1:]...)
 	var stdout bytes.Buffer