scm:
- Add methods to deal with git configs and tests for them.
  Will make it easier to mock git config calls as opposed
  to all git calls.
- Add tests to FetchUpstreamTuple

git-cl:
- Mock out settings.GetChangeRoot()
- Use scm.GIT.FetchUpstreamTuple() to reduce code duplication,
  and mock it out on tests.

Bug: 1051631
Change-Id: I1a3220b4c0f3e6221b3c00550259a433c2775798
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2052552
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>