git-sh-setup: use a lowercase "usage:" string

git-sh-setup: use a lowercase "usage:" string

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>