pull: pass --allow-unrelated-histories to "git merge"

pull: pass --allow-unrelated-histories to "git merge"

The previous commit said:

    We could add the same option to "git pull" and have it passed
    through to underlying "git merge".  I do not have a fundamental
    opposition against such a feature, but this commit does not do
    so and instead leaves it as low-hanging fruit for others,
    because such a "two project merge" would be done after fetching
    the other project into some location in the working tree of an
    existing project and making sure how well they fit together, it
    is sufficient to allow a local merge without such an option
    pass-through from "git pull" to "git merge".

Prepare a patch to make it a reality, just in case it is needed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>