init: add --manifest-upstream-branch

When a sha1 is provided to '--manifest-branch', the ref which
is expected to contain that sha1 can be provided using the new
'--manifest-upstream-branch' option. This is useful with
'--current-branch' to avoid having to sync all heads and tags,
or with a commit that comes from a non-head/tag ref (like a
Gerrit change ref).

Change-Id: I46a3e255ca69ed9e809039e58b0c163e02af94ef
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/436717


Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Kaushik Lingarkar <kaushik.lingarkar@linaro.org>
Tested-by: Kaushik Lingarkar <kaushik.lingarkar@linaro.org>