fix bugs in omniglot plugin and add example of hg repo

Note: don't merge this just yet; the test cases are empty.
How should we test that a link should point to a certain URL, or that the URL contains a certain string (commit hash)?

This fixes some bugs in the omniglot plugin as well as
providing a more useful view for the diff link.
Addresses [bugzilla #974535](https://bugzilla.mozilla.org/show_bug.cgi?id=974535)
Good catch; somebody hit that just the other day.