Fix support for renamed files, aka git mv in upload.py.
Fix support for renamed files, aka git mv in upload.py.

Adds usage of -C -C. With limited testing on the chromium tree of ~36k files, it
adds a few tens of percent to the time required to generate the diff.

Prior this change, a renamed file would only have the new file listed on
rietveld and not the previous one. This is caused by the default behavior of
"git diff".

This change calls "git diff" two times to extract all the necessary data.

BUG=251

Review: https://codereview.appspot.com/4333051/

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