checkpatch: avoid NOT_UNIFIED_DIFF errors on cover-letter.patch files

Make an exception for the "Does not appear to be a unified-diff" error
when scanning what appears to be git generated cover letters.
Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -5614,7 +5614,7 @@ sub process {
 		exit(0);
 	}
 
-	if (!$is_patch) {
+	if (!$is_patch && $file !~ /cover-letter\.patch$/) {
 		ERROR("NOT_UNIFIED_DIFF",
 		      "Does not appear to be a unified-diff format patch\n");
 	}