gitfs: accept per-remote auth params (fixes #16698)

This fix should be low-risk and I tested that it addresses the issue - that per-remote auth params cause an error which is contrary to the current documentation.

However, I didn't test that per-remote auth params actually work because I'm using GitPython, which doesn't support per-remote auth params.  A more thorough fix would be to log an error when the config has per-remote settings that aren't supported by whatever git library is being used.

So, feel free to merge this or close it in favor of a better fix in the future - I don't need it for my setup.