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

scm.py

Index: scm.py
diff --git a/scm.py b/scm.py
index eb5c52403c3e2595d14b133a7d0d00e289811d10..9bc96bc8055832707a749d3b9d256766b5a43956 100644
--- a/scm.py
+++ b/scm.py
@@ -353,11 +353,34 @@ class GIT(object):
     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