log: add "--no-show-signature" command line option

log: add "--no-show-signature" command line option

If an user creates an alias with "--show-signature" early in command
line, e.g.
	[alias] logss = log --show-signature

then there is no way to countermand it through command line.

Teach git-log and related commands about "--no-show-signature" command
line option. This will make "git logss --no-show-signature" run
without showing GPG signature.

Signed-off-by: Mehul Jain <mehul.jain2029@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>