Add simple changelog generator

    $ oc adm release info --changelog=/tmp/git ...

Will generate an opinionated changelog in markdown from the payload.
It requires `oc adm release extract --git=DIR` to be invoked first,
and will scan for PR merge commits in those repos and expose them.
Issues that begin with `Bug [0-9]+: ` will be linked to bugzilla.

This will eventually also be able to print a list of bugs that can
be marked as part of the payload.

pkg/oc/cli/admin/release/extract.go

@@ -12,6 +12,7 @@ import (
 	"path"
 	"path/filepath"
 	"regexp"
+	"strconv"
 	"strings"
 	"time"
 
@@ -240,16 +241,10 @@ func (o *ExtractOptions) extractGit(dir string) error {
 		}
 		alreadyExtracted[repo] = commit
 
-		u, err := sourceLocationAsURL(repo)
+		basePath, err := sourceLocationAsRelativePath(dir, repo)
 		if err != nil {
 			return err
 		}
-		gitPath := u.Path
-		if strings.HasSuffix(gitPath, ".git") {
-			gitPath = strings.TrimSuffix(gitPath, ".git")
-		}
-		gitPath = path.Clean(gitPath)
-		basePath := filepath.Join(dir, u.Host, filepath.FromSlash(gitPath))
 
 		var git *git
 		fi, err := os.Stat(basePath)
@@ -366,3 +361,107 @@ func sourceLocationAsURL(location string) (*url.URL, error) {
 	}
 	return url.Parse(location)
 }
+
+func sourceLocationAsRelativePath(dir, location string) (string, error) {
+	u, err := sourceLocationAsURL(location)
+	if err != nil {
+		return "", err
+	}
+	gitPath := u.Path
+	if strings.HasSuffix(gitPath, ".git") {
+		gitPath = strings.TrimSuffix(gitPath, ".git")
+	}
+	gitPath = path.Clean(gitPath)
+	basePath := filepath.Join(dir, u.Host, filepath.FromSlash(gitPath))
+	return basePath, nil
+}
+
+type MergeCommit struct {
+	CommitDate time.Time
+
+	Commit        string
+	ParentCommits []string
+
+	PullRequest int
+	Bug         int
+
+	Subject string
+}
+
+func mergeLogForRepo(g *git, from, to string) ([]MergeCommit, error) {
+	if from == to {
+		return nil, nil
+	}
+
+	rePR, err := regexp.Compile(`^Merge pull request #(\d+) from`)
+	if err != nil {
+		return nil, err
+	}
+	reBug, err := regexp.Compile(`^Bug (\d+): `)
+	if err != nil {
+		return nil, err
+	}
+
+	args := []string{"log", "--merges", "--topo-order", "-z", "--pretty=format:%H %P%x1E%ct%x1E%s%x1E%b", "--reverse", fmt.Sprintf("%s..%s", from, to)}
+	out, err := g.exec(args...)
+	if err != nil {
+		// retry once if there's a chance we haven't fetched the latest commits
+		if !strings.Contains(out, "Invalid revision range") {
+			return nil, fmt.Errorf(out)
+		}
+		if _, err := g.exec("fetch", "--all"); err != nil {
+			return nil, fmt.Errorf(out)
+		}
+		out, err = g.exec(args...)
+		if err != nil {
+			return nil, fmt.Errorf(out)
+		}
+	}
+
+	if glog.V(5) {
+		glog.Infof("Got commit info:\n%s", strconv.Quote(out))
+	}
+
+	var commits []MergeCommit
+	for _, entry := range strings.Split(out, "\x00") {
+		records := strings.Split(entry, "\x1e")
+		if len(records) != 4 {
+			return nil, fmt.Errorf("unexpected git log output width %d columns", len(records))
+		}
+		unixTS, err := strconv.ParseInt(records[1], 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("unexpected timestamp: %v", err)
+		}
+		commitValues := strings.Split(records[0], " ")
+
+		mergeCommit := MergeCommit{
+			CommitDate:    time.Unix(unixTS, 0).UTC(),
+			Commit:        commitValues[0],
+			ParentCommits: commitValues[1:],
+		}
+
+		msg := records[3]
+		if m := reBug.FindStringSubmatch(msg); m != nil {
+			mergeCommit.Subject = msg[len(m[0]):]
+			mergeCommit.Bug, err = strconv.Atoi(m[1])
+			if err != nil {
+				return nil, fmt.Errorf("could not extract bug number from %q: %v", msg, err)
+			}
+		} else {
+			mergeCommit.Subject = msg
+		}
+		mergeCommit.Subject = strings.TrimSpace(mergeCommit.Subject)
+
+		mergeMsg := records[2]
+		if m := rePR.FindStringSubmatch(mergeMsg); m != nil {
+			mergeCommit.PullRequest, err = strconv.Atoi(m[1])
+			if err != nil {
+				return nil, fmt.Errorf("could not extract PR number from %q: %v", mergeMsg, err)
+			}
+		}
+
+		commits = append(commits, mergeCommit)
+	}
+
+	return commits, nil
+}