scripts/get_maintainer.pl: Add -f directory use

Don't require a specific file in a directory to be tested.

Also Arnd Bergmann pointed out that the MAINTAINERS pattern requirement
that directory patterns have a trailing slash was unnecessary and was
likely to be error prone.  Removed that requirement.

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>