Replace some calls to die(usage_str) with usage(usage_str).

Replace some calls to die(usage_str) with usage(usage_str).

The only change in behaviour should be having a "usage: " prefix
on the output string rather than "fatal: ", and an exit code of
129 rather than 128.

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