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 serve killdaemons
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ python "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py