Add trivial cases covering CliGitAPIImpl submoduleUpdate
Increase coverage of submodule tests

There are not enough assertions in the test for it to be especially
useful as a test, but I assume it is better to call a method and not
assert its results than it is to leave the method uncalled.

Once I've understood submodules more clearly, I'll be able to add more
submodule tests and more assertions around those tests.
Improve exception message in fixSubmoduleUrls, add a related test