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