get_maintainer: add test for file in VCS

It's somewhat common for me to ask get_maintainer to tell me who maintains
a patch file rather than the files modified by the patch.

Emit a warning if using get_maintainer.pl -f <patchfile>

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/f63229c051567041819f25e76f49d83c6e4c0f71.camel@perches.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>