Add rb_git_remote_check_connection

This is useful if one needs to check if it is possible to fetch/push from/to the remote.

Basically this is the analogue to `git fetch --dry-run` and `git push --dry-run`.

I figured it would be better to implement as a separate function instead of the options to `rb_git_remote_fetch` and `rb_git_remote_push`. But I'm eager to have your feedback.