Fix skipping update for git dependency.

scm.py

Index: scm.py
diff --git a/scm.py b/scm.py
index 0b617c62aaedb3ef8abdad11b86e19513336a4a1..35d11a4c4a3530e099abfd57f0a4977953747af1 100644
--- a/scm.py
+++ b/scm.py
@@ -405,6 +405,12 @@ class GIT(object):
   @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