checkout: implement "@{-N}" shortcut name for N-th last branch

Implement a shortcut @{-N} for the N-th last branch checked out, that
works by parsing the reflog for the message added by previous
git-checkout invocations.  We expand the @{-N} to the branch name, so
that you end up on an attached HEAD on that branch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin-checkout.c b/builtin-checkout.c
index b5dd9c0..a3b69d6 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -361,8 +361,14 @@
 static void setup_branch_path(struct branch_info *branch)
 {
 	struct strbuf buf = STRBUF_INIT;
-	strbuf_addstr(&buf, "refs/heads/");
-	strbuf_addstr(&buf, branch->name);
+
+	if (!interpret_nth_last_branch(branch->name, &buf)) {
+		branch->name = xstrdup(buf.buf);
+		strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
+	} else {
+		strbuf_addstr(&buf, "refs/heads/");
+		strbuf_addstr(&buf, branch->name);
+	}
 	branch->path = strbuf_detach(&buf, NULL);
 }