commit with specified files does not go in lexicographical order

If you do a regular 'bzr commit' then we iterate the filesystem in sorted order.

However, if you do "bzr commit foo bar baz" then we put those paths into a set (based on osutils.minimum_path_selection()) and then iterate that set (via .pop()). As such, the output of what files are being committed is random.
Even further, it probably walks the fs in a random order, which could probably be better.

It would seem that the best place to fix this would be in _dirstate_helpers_pyx.pyx to use a sorted "search_specific_files" list, rather than a set. Even further, osutils.minimum_path_selection already builds a sorted list (to make it cheaper to determine if a path is in the minimal set already). So we possibly could just return that. Callers that need a set could then do the work at that level instead.