checkpatch: warn on bare SHA-1 commit IDs in commit logs

Commit IDs should have commit descriptions too.  Warn when a 12 to 40 byte
SHA-1 is used in commit logs.

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>