t5526: create superproject commits with test helper

t5526: create superproject commits with test helper

A few tests in t5526 use this pattern as part of their setup:

1. Create new commits in the upstream submodules (using
   add_upstream_commit()).
2. In the upstream superprojects, add the new submodule commits from the
   previous step.

A future commit will add more tests with this pattern, so reduce the
verbosity of present and future tests by introducing a test helper that
creates superproject commits. Since we now have two helpers that add
upstream commits, rename add_upstream_commit() to
add_submodule_commits().

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>