get_maintainer.pl: teach git log to use --no-color

When git has been set to always use color in .gitconfig then I get the
warning message

        Bad divisor in main::vcs_assign: 0

This is caused by vcs_file_signoffs not matching any commits due to the
pattern not understand the colour codes.  Fix this by telling git log to
never use colour.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>