submodule: check for unmerged .gitmodules outside of config parsing

submodule: check for unmerged .gitmodules outside of config parsing

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>