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: 4cad35a ("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>

scripts/get_maintainer.pl

@@ -432,7 +432,7 @@ sub read_mailmap {
 	    die "$P: file '${file}' not found\n";
 	}
     }
-    if ($from_filename || vcs_file_exists($file)) {
+    if ($from_filename || ($file ne "&STDIN" && vcs_file_exists($file))) {
 	$file =~ s/^\Q${cur_path}\E//;	#strip any absolute path
 	$file =~ s/^\Q${lk_path}\E//;	#or the path to the lk tree
 	push(@files, $file);