t7601: test interaction of merge/rebase/fast-forward flags and options

t7601: test interaction of merge/rebase/fast-forward flags and options

The interaction of rebase and merge flags and options was not well
tested.  Add several tests to check for correct behavior from the
following rules:
    * --ff-only vs. --[no-]rebase
      (and the related pull.ff=only vs. pull.rebase)
    * --rebase[=!false] vs. --no-ff and --ff
      (and the related pull.rebase=!false overrides pull.ff=!only)
    * command line flags take precedence over config, except:
      * --no-rebase heeds pull.ff=!only
      * pull.rebase=!false vs --no-ff and --ff

For more details behind these rules and a larger table of individual
cases, refer to https://lore.kernel.org/git/xmqqwnpqot4m.fsf@gitster.g/
and the links found therein.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>