cmd/coordinator: avoid ambiguous branch names

When trying to resolve a branch name, if there is a remote or tag
of the same name, git cannot resolve it and issues an warning that
"refname 'BRANCH' is ambiguous."  This was discovered in gccgo's
watcher process on the branch 'asan' which is also the name of a tag
in the gcc repo.  To disambiguate between a remote and a tag, any
refname can be prepended with "heads/" or "tags/", respectively.

Change-Id: I16679f5249a5f22df59ab3a0ed49459bbbf5367b
Reviewed-on: https://go-review.googlesource.com/16083
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/coordinator/watcher_process.go

@@ -328,7 +328,7 @@ func (r *Repo) postNewCommits(b *Branch) error {
 			}
 		} else {
 			// Find the commit that this branch forked from.
-			base, err := r.mergeBase(b.Name, master)
+			base, err := r.mergeBase("heads/"+b.Name, master)
 			if err != nil {
 				return err
 			}
@@ -460,19 +460,26 @@ func (r *Repo) update(noisy bool) error {
 		b := r.branches[name]
 
 		// Find all unseen commits on this branch.
-		revspec := name
+		revspec := "heads/" + name
 		if b != nil {
 			// If we know about this branch,
 			// only log commits down to the known head.
-			revspec = b.Head.Hash + ".." + name
+			revspec = b.Head.Hash + ".." + revspec
 		} else if name != master {
 			// If this is an unknown non-master branch,
 			// log up to where it forked from master.
-			base, err := r.mergeBase(name, master)
+			base, err := r.mergeBase(revspec, master)
+			if base == "" {
+				// This branch did not fork from master so we
+				// don't care about it.
+				delete(r.branches, name)
+				log.Printf("Found independent branch %s. This branch will not be watched.", name)
+				continue
+			}
 			if err != nil {
 				return err
 			}
-			revspec = base + ".." + name
+			revspec = base + ".." + revspec
 		}
 		log, err := r.log("--topo-order", revspec)
 		if err != nil {
@@ -610,7 +617,7 @@ func (r *Repo) mergeBase(a, b string) (string, error) {
 	cmd.Dir = r.root
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		return "", fmt.Errorf("git merge-base: %v", err)
+		return "", fmt.Errorf("git merge-base %s..%s: %v", a, b, err)
 	}
 	return string(bytes.TrimSpace(out)), nil
 }