checkpatch: fix filename detection when using -f

Fix $realfile filename when using -f/--file to not remove first level
directory as if the filename was used in a -P1 patch.  Only strip the
first level directory (typically a or b) for P1 patches.

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>
(extracted from Linux commit 2b7ab45395dc4d91ef30985f76d90a8f28f58c27)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/checkpatch.pl

@@ -1367,10 +1367,10 @@ sub process {
 		# extract the filename as it passes
 		if ($line =~ /^diff --git.*?(\S+)$/) {
 			$realfile = $1;
-			$realfile =~ s@^([^/]*)/@@;
+			$realfile =~ s@^([^/]*)/@@ if (!$file);
 		} elsif ($line =~ /^\+\+\+\s+(\S+)/) {
 			$realfile = $1;
-			$realfile =~ s@^([^/]*)/@@;
+			$realfile =~ s@^([^/]*)/@@ if (!$file);
 
 			$p1_prefix = $1;
 			if (!$file && $tree && $p1_prefix ne '' &&