Fixup command names in some usage strings.

Most usage strings, such as for command xxx, start with "git-xxx".
This updates the rebels to conform to the general pattern.
(The git wrapper is an exception to this, of course ...)

Signed-off-by: Ramsay Allan Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>