checkpatch: add test for commit id formatting style in commit log

Commit logs have various forms of commit id references.

Try to standardize on a 12 character long lower case commit id along
with a description of parentheses and the quoted subject line.

ie: commit 0123456789ab ("commit description")

If git and a git tree exists, look up the commit id and emit the
appropriate line as part of the message.

Signed-off-by: Joe Perches <joe@perches.com>
Requested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>