scripts/get_maintainer.pl: change "die" to "warn" when command line file is not a patch

fixes git send-email with a cover letter

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>