fix git submodule fetch output

Summary:
As described in issue #21, when we do submodule clones, we print the
(top) parent's url instead. As part of fixing this, get rid of the git plugin's
`FetchJob` class entirely.

Test Plan: Clone https://github.com/olson-sean-k/dot-config and do a sync.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D42