Add tryserver.api.gerrit_change_info that returns detailed info about the
current Gerrit CL. It makes a Gerrit RPC in the background on the first read.
This step will be present in all tryjobs.

This mechanism can be used for extra details about the current CL.
This CL starts to use this in bot_update for gerrit_ref, and stops using
patch_ref property, and also update bot_update to use when the destination
branch is needed.

Also this CL removes no longer used properties in bot_update,
namely most of patch_* properties.

build.git manual roll: https://chromium-review.googlesource.com/c/chromium/tools/build/+/1217626

Recipe-Manual-Change: build
Recipe-Nontrivial-Roll: infra
Recipe-Nontrivial-Roll: skia
Recipe-Nontrivial-Roll: skiabuildbot
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Recipe-Nontrivial-Roll: release_scripts
R=tandrii@chromium.org
Bug: 694348
Change-Id: I945d061b74278c3d1d227578abe16a64cdaca647
Reviewed-on: https://chromium-review.googlesource.com/1213557
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>