Fix skipping update for git dependency.

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 387 matching lines @@
     try:
       lines = GIT.Capture(
           ['svn', 'find-rev', 'r' + str(rev)], cwd=cwd).splitlines()
       return lines[-1].strip() if lines else None
     except subprocess2.CalledProcessError:
       return None
 
   @staticmethod
   def IsValidRevision(cwd, rev):
     """Verifies the revision is a proper git revision."""
+    # 'git rev-parse foo' where foo is *any* 40 character hex string will return
+    # the string and return code 0. So strip one character to force 'git
+    # rev-parse' to do a hash table look-up and returns 128 if the hash is not
+    # present.
+    if re.match(r'^[0-9a-fA-F]{40}$', rev):
+      rev = rev[:-1]
     try:
       GIT.Capture(['rev-parse', rev], cwd=cwd)
       return True
     except subprocess2.CalledProcessError:
       return False
 
   @classmethod
   def AssertVersion(cls, min_version):
     """Asserts git's version is at least min_version."""
     if cls.current_version is None:
@@ skipping 639 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