checkpatch.pl: don't complain about old lines with tabs

Don't complain when the patch includes lines with tabs
only in the hunk's untouched context.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

scripts/checkpatch.pl

@@ -1495,7 +1495,7 @@ sub process {
 		next if ($realfile !~ /\.(h|c|pl)$/);
 
 # in QEMU, no tabs are allowed
-		if ($rawline =~ /\t/) {
+		if ($rawline =~ /^\+.*\t/) {
 			my $herevet = "$here\n" . cat_vet($rawline) . "\n";
 			ERROR("code indent should never use tabs\n" . $herevet);
 			$rpt_cleaners = 1;