Add 'is_gitmodules_unmerged()' function which can be used to determine
in the '.gitmodules' file is unmerged based on the passed in index
instead of relying on a global variable which is set during the
submodule-config parsing.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>