Previously, git-cl-diff went through a dance where it would create
a new branch, download the uploaded patch onto that branch, and
then diff against that. This had all sorts of problems: if you
aborted the command, it might leave you on that branch; if you have
local changes, they might get clobbered or the command would refuse
to run.

Now that we're in a Gerrit-only world, and patchsets are by definition
equivalent to commits, we can simply diff against whatever local commit
was last uploaded or, in a pinch, fetch the uploaded commit and diff
against that.

Bug: 759893
Change-Id: Ia4b93dcfb9b8aba85817e62731f68d6450026e75
Reviewed-on: https://chromium-review.googlesource.com/639915Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>