gclient_scm.py: Make working with git more reliable

scm.py

Index: scm.py
diff --git a/scm.py b/scm.py
index 038d2dd4719a5ad777c383e6a83fc346eae9a9da..bd11394565b1e927f934a310de6ae67550521e3b 100644
--- a/scm.py
+++ b/scm.py
@@ -65,12 +65,16 @@ class GIT(object):
       raise
 
   @staticmethod
-  def CaptureStatus(files, upstream_branch='origin'):
+  def CaptureStatus(files, upstream_branch=None):
     """Returns git status.
 
     @files can be a string (one file) or a list of files.
 
     Returns an array of (status, file) tuples."""
+    if upstream_branch is None:
+      upstream_branch = GIT.GetUpstreamBranch(os.getcwd())
+      if upstream_branch is None:
+        raise Exception("Cannot determine upstream branch")
     command = ["diff", "--name-status", "-r", "%s..." % upstream_branch]
     if not files:
       pass