Add a shortcut for getting local path to the patch repo.

In bot_update.api._destination_branch, start using it instead of reading
gclient config directly. This removes one more patch_project usage.

Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: infra
Bug: 694348
Change-Id: If05371d0f6d11be6803ffa8392e029f5860dda25
Reviewed-on: https://chromium-review.googlesource.com/1217670
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>