More remote rename fixes

There is no reason why we need to use a callback here. A string array
fits better with the usage, as this is not an event and we don't need
anything from the user.

I wrote this stuff a while ago and forgot to write tests.  Wanted
to do so now to wrap up the PR and immediately found problems.

The base for the relative urls is determined as follows, with descending
priority:

- remote url of HEAD's remote tracking branch
- remote "origin"
- workdir

This follows git.git behaviour

There was a little bug where the submodule cache thought that the
index date was out of date even when it wasn't that was resulting
in some extra scans of index data even when not needed.

Mostly this commit adds a bunch of new tests including adding and
removing submodules in the index and in the HEAD and seeing if we
can automatically pick them up when refreshing.

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.