Show usage string for 'git cherry -h'

Show usage string for 'git cherry -h'

Treat an "-h" option as a request for help, rather than an
"Unknown commit -h" error.

"cherry -h" could be asking to compare histories that leads to
our HEAD and a commit that can be named as "-h".  Strictly
speaking, that may be a valid refname, but the user would have to
say something like "tags/-h" to name such a pathological ref
already, so it is not such a big deal.

The "-h" option keeps its meaning even if preceded by other
options or followed by other arguments.  This keeps the
command-line syntax closer to what parse_options would give and
supports shell aliases like 'alias cherry="git cherry -v"' a
little better.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>