mq: add qdelete --forget option
This removes an applied patch from the series and status files without
popping it. It is useful when an mq patch has been applied upstream.
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)