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
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev offset length ..... linkrev nodeid p1 p2 (re)
0 0 3 ..... 0 1e88685f5dde 000000000000 000000000000 (re)