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.

Wrote tests that try adding, removing, and updating the name of
submodules which showed a number of problems with how we account
for changes when incrementally updating the submodule info.  Most
of these issues didn't exist before because reloading would always
blow away the old submodule data.

The reload_all call could end up dereferencing a NULL pointer if
there was an error while attempting to load the submodules config
data (i.e. invalid content in the gitmodules file).  This fixes it.