Add support for --progress when updating submodules

[therocode]

When using the class UpdateProgress to monitor the progress of submodules updating, you don't get a lot of progress from the CLONE step (it only has a max_count of 1) and for some submodules this takes a really long time, meaning it is hard to make a good UX for this case.

I assume the progress is not reported in more detail due to how the progress is not shown when using git itself on the commandline. However, there is now a flag for this in git -> https://stackoverflow.com/a/48283379/2164642

This sounds to me like it would be possible to improve the progress reporting of submodules to be a lot more informative similar to the normal top-level cloning.

[Byron]

Thanks for the description! It should be relatively straightforward to pass flags down to git-clone from the submodule implementation. Maybe it's already possible, most methods support **kwargs for that purpose.