-F/--force-rebase has no effect if rebase is disabled by config

--force-rebase has no effect if rebase is disabled through git-config
or git-review config files.

This change ensures -F/--force-rebase "wins" over rebase disabled and
-F/--force-rebase becomes mutually exclusive with -R/--no-rebase.

Change-Id: Idd985a6a7a5b1cffa9bed996bb1ea5907e7d0844
Merge "-F/--force-rebase has no effect if rebase is disabled by config"