dontdiff: update based on gitignore updates

Update dontdiff, based on .gitignore patches from Pete Zaitcev and Adrian
Bunk.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Cc: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>