Fix gclient branch ref mangling and allow --force branch switches.

scm.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """SCM-specific utility classes."""
 
 import cStringIO
 import glob
 import logging
 import os
@@ skipping 335 matching lines @@
             # git-svn clone
             remote = 'origin'
             upstream_branch = 'refs/heads/trunk'
           else:
             # Give up.
             remote = None
             upstream_branch = None
     return remote, upstream_branch
 
   @staticmethod
+  def RefToRemoteRef(ref, remote=None):
+    """Convert a checkout ref to the equivalent remote ref.
+
+    Returns:
+      A tuple of the remote ref's (common prefix, unique suffix), or None if it
+      doesn't appear to refer to a remote ref (e.g. it's a commit hash).
+    """
+    # TODO(mmoss): This is just a brute-force mapping based of the expected git
+    # config. It's a bit better than the even more brute-force replace('heads',
+    # ...), but could still be smarter (like maybe actually using values gleaned
+    # from the git config).
+    m = re.match('^(refs/(remotes/)?)?branch-heads/', ref or '')
+    if m:
+      return ('refs/remotes/branch-heads/', ref.replace(m.group(0), ''))
+    if remote:
+      m = re.match('^((refs/)?remotes/)?%s/|(refs/)?heads/' % remote, ref or '')
+      if m:
+        return ('refs/remotes/%s/' % remote, ref.replace(m.group(0), ''))
+    return None
+
+  @staticmethod
   def GetUpstreamBranch(cwd):
     """Gets the current branch's upstream branch."""
     remote, upstream_branch = GIT.FetchUpstreamTuple(cwd)
     if remote != '.' and upstream_branch:
-      upstream_branch = upstream_branch.replace('heads', 'remotes/' + remote)
+      remote_ref = GIT.RefToRemoteRef(upstream_branch, remote)
+      if remote_ref:
+        upstream_branch = ''.join(remote_ref)
     return upstream_branch
 
   @staticmethod
   def GenerateDiff(cwd, branch=None, branch_head='HEAD', full_move=False,
                    files=None):
     """Diffs against the upstream branch or optionally another branch.
 
     full_move means that move or copy operations should completely recreate the
     files, usually in the prospect to apply the patch for a try job."""
     if not branch:
@@ skipping 766 matching lines @@
         # revert, like for properties.
         if not os.path.isdir(cwd):
           # '.' was deleted. It's not worth continuing.
           return
         try:
           SVN.Capture(['revert', file_status[1]], cwd=cwd)
         except subprocess2.CalledProcessError:
           if not os.path.exists(file_path):
             continue
           raise