- Instead of expecting the caller to run `rev-parse HEAD` before
  calling gclient, we can just store the latest commit on disk
  - this gets around the problem where gclient might git reset to the
     base checkout after applying patch-refs, so calling rev-parse
     before calling gclient sync might not actually return the latest
     commit of the last sync.
- also moving os.environ[PREVIOUS_CUSTOM_VARS] setting to earlier when
  we only have the solutions in self.dependencies because all other
  dependencies inherit from the solutions so there's no point storing
  custom_vars for the rest.

Bug: 1339472
Change-Id: I6a3570f09153bd8087bbe6bdab7ece4949856aae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3750491Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Joanna Wang <jojwang@chromium.org>