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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username
foobar
#endif