presubmit: Call 'git diff' once per change instead of once per file

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index 082b4ebff80196f73684b3b512a2f09691f1e5ad..728350c50e840a02317c6c1588d151f02267ab5a 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -696,9 +696,9 @@ or verify this branch is set up to track another (via the --track argument to
           ('\nFailed to diff against upstream branch %s!\n\n'
            'This branch probably doesn\'t exist anymore. To reset the\n'
            'tracking branch, please run\n'
-           '    git branch --set-upstream %s trunk\n'
+           '    git branch --set-upstream-to=trunk\n'

[iannucci, 2013/06/07 21:30:16]

This may be the old syntax because some users still have 'old' git versions.
Could you perhaps have both flavors in the message?

[ncarter (slow), 2013/06/10 19:28:46]

I think this is a case where adding more text only creates more confusion. Since
this feature isn't in git 1.7, I reverted to the old instructions.

            'replacing trunk with origin/master or the relevant branch') %
-          (upstream_branch, self.GetBranch()))
+          (upstream_branch))
 
     issue = self.GetIssue()
     patchset = self.GetPatchset()