Add a `--rebase` option to sync command

Previously repo would abort a sync if there were published changes not
merged upstream. The --rebase option allows the modification of
published commits.

This is a copy of http://go/grev/369694 with the merge conflicts
resolved.

Bug: 40014610
Change-Id: Idac8199400346327b530abea33f1ed794e5bb4c2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/435838


Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>