Add helpers to gerrit_util for:

  * Setting the commit message on a ChangeEdit,
  * Creating a cherry-pick,
  * Getting the contents of a file in a change.

Bug: v8:12849
Change-Id: I1a5de3054a366d480def32942fd42be179ffb749
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3683383
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>