The options added to __git_merge_options are those that git-pull passes
to git-merge, since that variable is used by both commands.

Those added directly in _git_merge() are specific to git-merge and
are not passed thru from git-pull.
Reported-by: NHaralan Dobrev <hkdobrev@gmail.com>
Signed-off-by: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>