get_maintainer.pl: Add git-min-percent option

Allow an option to control the minimum percentage of sign-offs required
before being considered a maintainer.

git-min-percent has a default value of 5

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>