Allow relative path url in submodules for submodule_update

[t-webber]

fixes #944

[Byron]

Thanks a lot for contributing!

I think with a test this can be merged. Maybe looking at the existing tests for submodules can help to set one up.

Thanks again.

[Byron]

Thanks a lot, this is great!

Something I'd like to also know is what happens if the submodule is 'hidden', and the URL starts with a .. This would mean the submodule resides within the parent repository, but is added as submodule under a different path.
Is this is a thing even? If you think so, please do feel free to contribute the respective test as well, but let's not block this PR on such an edge-case.

[t-webber]

I am not sure I understand your question.

When you run git add ../../other/repo without specifying the path, git adds a default of repo as path in .gitmodules, so still in the same folder.

If you try and remove the path argument, you get an error when trying to update the submodule. You have the same error if the submodule has an absolute path. Git seems to always want a path and a URL field in the .gitmodules file.

[Byron]

Please don't mind it, it's not too important I think.

Thanks again for contributing this fix and a test🙏.