Remove cmd_usage() routine and re-organize the help/usage code.

Remove cmd_usage() routine and re-organize the help/usage code.

The cmd_usage() routine was causing warning messages due to a NULL
format parameter being passed in three out of four calls. This is a
problem if you want to compile with -Werror. A simple solution is to
simply remove the GNU __attribute__ format pragma from the cmd_usage()
declaration in the header file. The function interface was somewhat
muddled anyway, so re-write the code to finesse the problem.

[jc: this incidentally revealed that t9100 test assumed that the output
 from "git help" to be fixed in stone, but this patch lower-cases
 "Usage" to "usage".  Update the test not to rely on "git help" output.]

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