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

@@ -240,16 +240,10 @@ 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)
@@ -341,14 +335,14 @@ func (g *git) basename() string {
 }
 
 func (g *git) CheckoutCommit(repo, commit string) error {
-	_, err := g.exec("rev-parse", commit)
+	_, err := g.exec("show-ref", commit)
 	if err == nil {
 		return nil
 	}
 
 	// try to fetch by URL
 	if _, err := g.exec("fetch", repo); err == nil {
-		if _, err := g.exec("rev-parse", commit); err == nil {
+		if _, err := g.exec("show-ref", commit); err == nil {
 			return nil
 		}
 	}
@@ -366,3 +360,17 @@ 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
+}