Added checks:
- Block upload of a diff from sha not in current branch
  (this includes tracking against an unrebased local master)
- Block upload of a diff containing shas from origin/master
  (should never happen)

- Use explicit calls to git merge-base instead of calling git diff
   with "<branch>..." which implicitly uses merge-base.

BUG=157503

Review URL: https://chromiumcodereview.appspot.com/11262057

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@165887 0039d316-1c4b-4281-b951-d872f2087c98