git_handler: use `self.ui.paths` instead of directly interpreting config

This correctly handles sub-options. Before, the test case failed with: dulwich.errors.RefFormatError: refs/remotes/default:pushurl/beta

Based on !35 (merged).

Found in https://bz.mercurial-scm.org/show_bug.cgi?id=6274, not yet reported in our bugtracker.