Make webkit-patch pretty-diff do the right thing for upstream branchs.
Make webkit-patch pretty-diff do the right thing for upstream branchs.

If your current branch has an upstream branch, then diff to that
branch. That way you get the same diff that git cl upload gives
instead of the diff to origin/master.

TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/948663002

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