extensions: prohibit registration of command without using @command (API)
Detect the problem earlier for better error indication. I'm tired of teaching
users that the mq extension is not guilty but the third-party extension is.
https://bitbucket.org/tortoisehg/thg/issues?q=%27norepo%27
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors