This is a reland of https://crrev.com/c/6168707 with fixes.
The CL adds a new param, unified, to git_diff() and create_diffs()
as a required param.
The functions themselves are tested in presubmit_diffs_test.py, but
their usages in presubmit_support.py were not, and the missing param
caused a failure in CiderG (b/389876151)
This CL basically carries the same patch, but makes the params
optional with a default value and adds unit tests for the usage
in presubmit_support.py
Tested with CiderG: https://screenshot.googleplex.com/PYEDZ3NGn5cYGtV
Change-Id: Ic747c6a133c016dbcd80034e521c76a3db3a3f60
Bug: 389876151, 379902295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6182703
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Gary Tong <gatong@chromium.org>
