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
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.