This CL wholly revamps the way presubmit_support adds
CQ_INCLUDE_TRYBOTS lines in commit descriptions. In
particular, when the CL is being uploaded to Gerrit,
it uses our pre-existing support for manipulating git
footers to make the whole process much simpler.

R=tandrii@chromium.org

Bug: 710547
Change-Id: I5858282a44c590f131021fa3820f1cb3f70ef620
Reviewed-on: https://chromium-review.googlesource.com/487831
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>