Fix some repository URI handling issues in Git and Mercurial

Summary:
See <https://github.com/facebook/phabricator/issues/467>. @dctrwatson also ran into an issue where we were trying to `setPass()` a GitURI.

  - For Git and Mercurial, properly generate credential URIs where relevant.
  - Don't try to `setPass()` on Git-style URIs.

This isn't perfect but should clean things up a bit.

Test Plan: Added unit tests. Lots of `grep`.

Reviewers: btrahan

Reviewed By: btrahan

CC: dctrwatson, aran

Differential Revision: https://secure.phabricator.com/D7759