xdiff: print post-image for common records instead of pre-image

Normally it doesn't matter if we show the pre-image or th post-image
for the common parts of a diff because they are the same.  If
white-space changes are ignored they can differ, though.  The
new text after applying the diff is more interesting in that case,
so show that instead of the old contents.

Note: GNU diff shows the pre-image.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -103,7 +103,7 @@ test_expect_success 'another test, with -w --ignore-space-at-eol' 'test_cmp expe
 git diff -w -b --ignore-space-at-eol > out
 test_expect_success 'another test, with -w -b --ignore-space-at-eol' 'test_cmp expect out'
 
-tr 'Q' '\015' << EOF > expect
+tr 'Q_' '\015 ' << EOF > expect
 diff --git a/x b/x
 index d99af23..8b32fb5 100644
 --- a/x
@@ -111,19 +111,19 @@ index d99af23..8b32fb5 100644
 @@ -1,6 +1,6 @@
 -whitespace at beginning
 +	whitespace at beginning
- whitespace change
+ whitespace 	 change
 -whitespace in the middle
 +white space in the middle
- whitespace at end
+ whitespace at end__
  unchanged line
- CR at endQ
+ CR at end
 EOF
 git diff -b > out
 test_expect_success 'another test, with -b' 'test_cmp expect out'
 git diff -b --ignore-space-at-eol > out
 test_expect_success 'another test, with -b --ignore-space-at-eol' 'test_cmp expect out'
 
-tr 'Q' '\015' << EOF > expect
+tr 'Q_' '\015 ' << EOF > expect
 diff --git a/x b/x
 index d99af23..8b32fb5 100644
 --- a/x
@@ -135,9 +135,9 @@ index d99af23..8b32fb5 100644
 +	whitespace at beginning
 +whitespace 	 change
 +white space in the middle
- whitespace at end
+ whitespace at end__
  unchanged line
- CR at endQ
+ CR at end
 EOF
 git diff --ignore-space-at-eol > out
 test_expect_success 'another test, with --ignore-space-at-eol' 'test_cmp expect out'