presubmit_support: Call git once per change, to get a diff for all files. Parse this result to generate per-file diffs, which go into a cache that's shared between the AffectedFile instances.

Greatly improves presubmit performance on Blink where there may be rule violations (my test case went from 50s to 5s).

BUG=236206
TEST=new test in presubmit_unittest.py; manual performance test on Mac and Windows after touching hundreds of files in webkit; testing included add/move/edit/delete on both binary and text files.

Review URL: https://chromiumcodereview.appspot.com/15898005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@205275 0039d316-1c4b-4281-b951-d872f2087c98