Fix prune when bare git has detached head

We don't really use HEAD much in the bare git repositories, but there
have been reports of errors in git-symbolic-ref:

  symbolic-ref: fatal: Refusing to point HEAD outside of refs/

That happen when the bare git repo is in the detached head state. It's
possible that previous operations were killed while we were pruning
branches.

Use DetachHead instead of SetHead if we're restoring the repo into a
detached head state.

Change-Id: I9062e8957bc70367d3ded399685ac026fbb421fc

project.py

@@ -1583,8 +1583,6 @@ class Project(object):
 
     if kill:
       old = self.bare_git.GetHead()
-      if old is None:
-        old = 'refs/heads/please_never_use_this_as_a_branch_name'
 
       try:
         self.bare_git.DetachHead(rev)
@@ -1596,7 +1594,10 @@ class Project(object):
                        capture_stderr=True)
         b.Wait()
       finally:
-        self.bare_git.SetHead(old)
+        if ID_RE.match(old):
+          self.bare_git.DetachHead(old)
+        else:
+          self.bare_git.SetHead(old)
         left = self._allrefs
 
       for branch in kill: