The function was removed, but its declaration and changelog entry about
its removal were forgotten.

The comment in the test doesn't make any sense as the function doesn't
exist anymore, so get rid of it as well.

Having this cache and giving them out goes against our multithreading
guarantees and it makes it impossible to use submodules in a
multi-threaded environment, as any thread can ask for a refresh which
may reallocate some string in the submodule struct which we've accessed
in a different one via a getter.

This makes the submodules behave more like remotes, where each object is
created upon request and not shared except explicitly by the user. This
means that some tests won't pass yet, as they assume they can affect the
submodule objects in the cache and that will affect later operations.

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.