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>