Do not list twice patches in .changes entry

Right now, we get something like:

- Rebased patches:
  + patches/hide-unneeded-options.patch (only offset)
  + hide-unneeded-options.patch (manually)

This is obviously wrong.

This is using os.path.basename instead of changing the regexp, to be on
the totally safe side in case there's another path than patches/ for
quilt.