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

@@ -103,27 +103,27 @@ 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
 +++ b/x
 @@ -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'

xdiff/xemit.c

@@ -87,7 +87,7 @@ static long def_ff(const char *rec, long len, char *buf, long sz, void *priv)
 
 static int xdl_emit_common(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
                            xdemitconf_t const *xecfg) {
-	xdfile_t *xdf = &xe->xdf1;
+	xdfile_t *xdf = &xe->xdf2;
 	const char *rchg = xdf->rchg;
 	long ix;
 
@@ -151,16 +151,16 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		/*
 		 * Emit pre-context.
 		 */
-		for (; s1 < xch->i1; s1++)
-			if (xdl_emit_record(&xe->xdf1, s1, " ", ecb) < 0)
+		for (; s2 < xch->i2; s2++)
+			if (xdl_emit_record(&xe->xdf2, s2, " ", ecb) < 0)
 				return -1;
 
 		for (s1 = xch->i1, s2 = xch->i2;; xch = xch->next) {
 			/*
 			 * Merge previous with current change atom.
 			 */
 			for (; s1 < xch->i1 && s2 < xch->i2; s1++, s2++)
-				if (xdl_emit_record(&xe->xdf1, s1, " ", ecb) < 0)
+				if (xdl_emit_record(&xe->xdf2, s2, " ", ecb) < 0)
 					return -1;
 
 			/*
@@ -186,8 +186,8 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		/*
 		 * Emit post-context.
 		 */
-		for (s1 = xche->i1 + xche->chg1; s1 < e1; s1++)
-			if (xdl_emit_record(&xe->xdf1, s1, " ", ecb) < 0)
+		for (s2 = xche->i2 + xche->chg2; s2 < e2; s2++)
+			if (xdl_emit_record(&xe->xdf2, s2, " ", ecb) < 0)
 				return -1;
 	}