use -h for synopsis and --help for manpage consistently

use -h for synopsis and --help for manpage consistently

A few scripted Porcelain implementations pretend as if the routine to show
their own help messages are triggered upon "git cmd --help", but a command
line parser of "git" will hijack such a request and shows the manpage for
the cmd subcommand.

Leaving the code to handle such input is simply misleading.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>