t5520: test pulling an octopus into an unborn branch

t5520: test pulling an octopus into an unborn branch

The code comment for "git merge" in builtin/merge.c, we say

    If the merged head is a valid one there is no reason
    to forbid "git merge" into a branch yet to be born.
    We do the same for "git pull".

and t5520 does have an existing test for that behaviour.  However,
there was no test to make sure that 'git pull' to pull multiple
branches into an unborn branch must fail.

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