submit_try: create the diff file using git, rather than relying on trychange.py

BUG=skia:1862

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R=epoger@google.com

Review URL: https://codereview.chromium.org/128413004

git-svn-id: http://skia.googlecode.com/svn/trunk@12967 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed