[cw-ctl] fix help message for schemadiff In global help (closes #2888538)
The leading space make it appears on multiple line. The help content is a bit
improved in the process.
usr/lib/python2.4/site-packages/cubicweb/
etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances