Only call IsValidRevision() when it is a git hash.

gclient_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.
 
 """Gclient-specific SCM-specific operations."""
 
 import logging
 import os
 import posixpath
 import re
@@ skipping 335 matching lines @@
       upstream_branch = scm.GIT.GetUpstreamBranch(self.checkout_path)
       if not upstream_branch or not upstream_branch.startswith('refs/remotes'):
         current_type = "hash"
         logging.debug("Current branch is not tracking an upstream (remote)"
                       " branch.")
       elif upstream_branch.startswith('refs/remotes'):
         current_type = "branch"
       else:
         raise gclient_utils.Error('Invalid Upstream: %s' % upstream_branch)
 
-    if not scm.GIT.IsValidRevision(cwd=self.checkout_path, rev=revision):
+    if (not re.match(r'^[0-9a-fA-F]{40}$', revision) or
+        not scm.GIT.IsValidRevision(cwd=self.checkout_path, rev=revision)):
       # Update the remotes first so we have all the refs.
       backoff_time = 5
       for _ in range(10):
         try:
           remote_output = scm.GIT.Capture(
               ['remote'] + verbose + ['update'],
               cwd=self.checkout_path)
           break
         except subprocess2.CalledProcessError, e:
           # Hackish but at that point, git is known to work so just checking for
@@ skipping 827 matching lines @@
       new_command.append('--force')
       if command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
         new_command.extend(('--accept', 'theirs-conflict'))
     elif options.manually_grab_svn_rev:
       new_command.append('--force')
       if command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
         new_command.extend(('--accept', 'postpone'))
     elif command[0] != 'checkout' and scm.SVN.AssertVersion('1.6')[0]:
       new_command.extend(('--accept', 'postpone'))
     return new_command