Fixed a clang plugin filter description that failed to have Markup wrapping it.
Surface indirect overrides in "overrides" queries. Ref bug 1030312.

Build the inverse of the overriddens graph during the whole-program pass, and look at it when generating needles.
Remove unused imports.
Make Config no longer dependent on filesystem objects. Ref bug 959728.

This saves us a file write during SingleFileTestCases.

build_instance will likely change to take a dict-like object later, making it even more manipulable, once we've converted to configobj.
Fix config bug wherein specifying `*` for `enabled_plugins` or `disabled_plugins` wouldn't take all the plugins into account.

Also remove the last vestiges of a single plugin folder. You'll need to remove references to plugin_folder in your config file if you had them and instead register your third-party plugins using setuptools entrypoints, as described at http://dxr.readthedocs.org/en/es/development.html#registration.
Fix bugs in omniglot plugin and add example of hg repo

This fixes some bugs in the omniglot plugin as well as
providing a more useful view for the diff link.
ref bug #974535: https://bugzilla.mozilla.org/show_bug.cgi?id=974535