mergetool, bisect, and other commands that use
git-sh-setup print a usage string that is inconsistent
with the rest of Git when they are invoked as "git $cmd -h".
The compiled builtins use the lowercase "usage:" string
but these commands say "Usage:". Adjust the shell library
to make these consistent.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>