cmdutil: drop obsolete comment about updating to '.'
Obsolete since
cf68e2649e0a (status: advertise --abort instead of
'update -C .' to abort a merge, 2018-08-09) and
5b04b6204931 (status:
advertise --abort instead of 'update -C .' to abort graft, 2018-08-09).
Differential Revision: https://phab.mercurial-scm.org/D5694
mq: slightly simplify check for patched working copy
Differential Revision: https://phab.mercurial-scm.org/D5693