checkpatch: add support to check already applied git commits

It's sometimes useful to scan already committed patches.

Add --git <revision range> to scan specific or multiple commits.

Single commits are scanned with
	--git <rev>
Multiple commits are scanned with
	--git <range>
	--git <commit>-<count>

[joe@perches.com:
o Don't exec git for each <commit>-<count>,
  use a single "git log -<count> <commit>"
o Consolidate the git exec for the <range> and <commit>-<count> variants
o Output 12 character commit hash ids
o Don't scan git commit merges
o Use -M to reduce the size of rename commits]

Signed-off-by: "Du, Changbin" <changbin.du@intel.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>