Previously, anything not equal to HEAD meant pinned revision.
This backfired on values like "origin/master" which are meant as
equivalent to HEAD, but which bot_update tried to ensure exist in
checkout, which was refreshed several times under assumption that
this is eventual consistency of git servers.

Instead, this CL treats repo as pinned if and only if the treeish
of the repo is matching sha1 regex.

R=hinoka@chromium.org

Bug: 838292
Change-Id: I9cb46535f46b2898cf54900ede7960ba08770d77
Post-Scriptum: git bisect + Led is awesome, took only 8 manual steps.
Reviewed-on: https://chromium-review.googlesource.com/1035793Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>