checkpatch: reduce number of `git log` calls with --git

checkpatch currently calls git log multiple times to first get the
<revision range> sha1 values and again to get the subject for each
individual sha1 commit.

Always get the sha1 and subject at the same time instead.  Store the
subject in a sha1 hash to avoid the second git log exec.

Link: http://lkml.kernel.org/r/274efab2332ad2308ab5de85a95d255f6e2de5f3.1462711962.git.joe@perches.com


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>