shell-scripts usage(): consistently exit with non-zero

shell-scripts usage(): consistently exit with non-zero

Earlier conversion of shell scripts to parse-options made usage()
to run "git cmd -h" which in turn emit LONG_USAGE and exit with 0
status.  This is inconsistent with the scripts that do not use
parse-options, whose usage() died with the message, exiting with 1.

Signed-off-by: Junio C Hamano <gitster@pobox.com>