submodule: fetch missing objects from default remote

submodule: fetch missing objects from default remote

When be76c21282 (fetch: ensure submodule objects fetched, 2018-12-06)
added support for fetching a missing submodule object by id, it
hardcoded the remote name as "origin" and deferred anything more
complicated for a later patch. Implement the NEEDSWORK item to remove
the hardcoded assumption by adding and using a submodule helper subcmd
'get-default-remote'. Fixing this lets 'git fetch --recurse-submodules'
succeed when the fetched commit(s) in the superproject trigger a
submodule fetch, and that submodule's default remote name is not
"origin".

Add non-"origin" remote tests to t5526-fetch-submodules.sh and
t5572-pull-submodule.sh demonstrating this works as expected and add
dedicated tests for get-default-remote.

Signed-off-by: Nasser Grainawi <nasser.grainawi@oss.qualcomm.com>
Reviewed-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>