get_maintainer: Don't check if STDIN exists in a VCS repository

If get_maintainer is not given any filename arguments on the command line,
the standard input is read for a patch.

But checking if a VCS has a file named &STDIN is not a good idea and fails.

Verify the nominal input file is not &STDIN before checking the VCS.

Fixes: 4cad35a7 ("get_maintainer.pl: reduce need for command-line option -f")
Reported-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>