query: use fn:contains instead of fn:starts-with for path matching

This is all sorts of wrong conceptually, but we have a combination of
[1] and [2], which makes it hard to use anything else.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=666749
[2] https://bugzilla.gnome.org/show_bug.cgi?id=684257