bugzilla: keep bug IDs in set. Bug IDs are collected into a set, and then silently converted in filter_real_bug_ids() into a list. For consistency, keep them in a set throughout and update the docstrings to say that.

bugzilla: rename filter_unknown_bug_ids to reflect its actual purpose. The method really removes from the list of bugs any that already have comment text referencing the changeset in question. Rename it to filter_cset_known_bug_ids().

test-paths.t: 'file:' disables [paths] entries for clone dest