Create a script to create a unified git diff of a local workspace
against a remote Gerrit commit. This will be used to create a diff file
for running presubmits.
Bug: b/323243527
Change-Id: Ia0d287624162dbfe5231a5653c9a962a6c85c8e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5374833
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>