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.
% init
% commit
adding a
% annotate -c
8435f90966e4: a
% annotate -d
Thu Jan 01 00:00:01 1970 +0000: a
% annotate -n
0: a
% annotate -u
nobody: a
% annotate -cdnu
nobody 0 8435f90966e4 Thu Jan 01 00:00:01 1970 +0000: a