With the new submodule cache validity checks, we generally don't
need to call git_submodule_reload_all to have up-to-date submodule
data.  Some tests are still calling it where I want to actually
test that it can be called safely and doesn't break anything, but
mostly it is not needed.

This also expands some of the existing submodule tests to cover
some variants on the behavior that was already being tested.