get_maintainer: Prepare for separate MAINTAINERS files

Allow for MAINTAINERS to become a directory and if it is,
read all the files in the directory for maintained sections.

Optionally look for all files named MAINTAINERS in directories
excluding the .git directory by using --find-maintainer-files.

This optional feature adds ~.3 seconds of CPU on an Intel
i5-6200 with an SSD.

Miscellanea:

 - Create a read_maintainer_file subroutine from the existing code
 - Test only the existence of MAINTAINERS, not whether it's a file

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