feature suggestion: optimize common parts for checkout --conflict=diff3

Hello,

Here comes another recipe for a different suggestion:

	git init
	echo 1 > file
	git add file
	git commit -m 'base'
	git branch branch
	seq 1 30 | grep -v 15 > file
	git commit -m 'add 2-30 without 15' file
	git checkout branch
	seq 1 30 | grep -v 16 > file
	git commit -m 'add 2-30 without 16' file
	git merge master
	git diff

This yields:

	diff --cc file
	index a07e697,5080129..0000000
	--- a/file
	+++ b/file
	@@@ -12,7 -12,7 +12,11 @@@
	  12
	  13
	  14
	++<<<<<<< HEAD
	 +15
	++=======
	+ 16
	++>>>>>>> master
	  17
	  18
	  19

as expected; nice and sweet. After

	git checkout --conflict=diff3 file

however the difference isn't that easy to spot any more. I expected

	diff --cc file
	index a07e697,5080129..0000000
	--- a/file
	+++ b/file
	@@@ -12,7 -12,7 +12,12 @@@
	  12
	  13
	  14
	++<<<<<<< ours
	 +15
	++||||||| base
	++=======
	+ 16
	++>>>>>>> theirs
	  17
	  18
	  19

But instead I get

	diff --cc file
	index a07e697,5080129..0000000
	--- a/file
	+++ b/file
	@@@ -1,29 -1,29 +1,61 @@@
	  1
	++<<<<<<< ours
	 +2
	 +3
	 +4
	 +5
	 +6
	 +7
	 +8
	 +9
	 +10
	 +11
	 +12
	 +13
	 +14
	 +15
	 +17
	 +18
	 +19
	 +20
	 +21
	 +22
	 +23
	 +24
	 +25
	 +26
	 +27
	 +28
	 +29
	 +30
	++||||||| base
	++=======
	+ 2
	+ 3
	+ 4
	+ 5
	+ 6
	+ 7
	+ 8
	+ 9
	+ 10
	+ 11
	+ 12
	+ 13
	+ 14
	+ 16
	+ 17
	+ 18
	+ 19
	+ 20
	+ 21
	+ 22
	+ 23
	+ 24
	+ 25
	+ 26
	+ 27
	+ 28
	+ 29
	+ 30
	++>>>>>>> theirs

Of course this is technically correct, just not maximally helpful.

Is this a missing optimisation for the diff3 case or did I miss a detail
that makes my expectation wrong?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |