Add GetModifiedFiles() function to git_utils.

It will be used in a follow up CL to replace _GitGetModifiedFiles() from
tools/svndiff.py in skia.googlesource.com/skia repo.

BUG=None
TEST=./py/run_unittests
R=borenet@google.com

Review URL: https://codereview.chromium.org/638723002