xdiff: unignore changes in function context

Changes involving only blank lines are hidden with --ignore-blank-lines,
unless they appear in the context lines of other changes.  This is
handled by xdl_get_hunk() for context added by --inter-hunk-context, -u
and -U.

Function context for -W and --function-context added by xdl_emit_diff()
doesn't pay attention to such ignored changes; it relies fully on
xdl_get_hunk() and shows just the post-image of ignored changes
appearing in function context.  That's inconsistent and confusing.

Improve the result of using --ignore-blank-lines and --function-context
together by fully showing ignored changes if they happen to fall within
function context.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -2008,11 +2008,6 @@ test_expect_success 'compare mixed whitespace delta across moved blocks' '
 	test_cmp expected actual
 '
 
-# Note that the "6" in the expected hunk header below is funny, since we only
-# show 5 lines (the missing one was blank and thus ignored). This is how
-# --ignore-blank-lines behaves even without --function-context, and this test
-# is just checking the interaction of the two features. Don't take it as an
-# endorsement of that output.
 test_expect_success 'combine --ignore-blank-lines with --function-context' '
 	test_write_lines 1 "" 2 3 4 5 >a &&
 	test_write_lines 1    2 3 4   >b &&
@@ -2022,6 +2017,7 @@ test_expect_success 'combine --ignore-blank-lines with --function-context' '
 	cat <<-\EOF >expect &&
 	@@ -1,6 +1,4 @@
 	 1
+	-
 	 2
 	 3
 	 4

xdiff/xemit.c

@@ -172,10 +172,12 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 	struct func_line func_line = { 0 };
 
 	for (xch = xscr; xch; xch = xche->next) {
+		xdchange_t *xchp = xch;
 		xche = xdl_get_hunk(&xch, xecfg);
 		if (!xch)
 			break;
 
+pre_context_calculation:
 		s1 = XDL_MAX(xch->i1 - xecfg->ctxlen, 0);
 		s2 = XDL_MAX(xch->i2 - xecfg->ctxlen, 0);
 
@@ -212,6 +214,21 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 			if (fs1 < s1) {
 				s2 = XDL_MAX(s2 - (s1 - fs1), 0);
 				s1 = fs1;
+
+				/*
+				 * Did we extend context upwards into an
+				 * ignored change?
+				 */
+				while (xchp != xch &&
+				       xchp->i1 + xchp->chg1 <= s1 &&
+				       xchp->i2 + xchp->chg2 <= s2)
+					xchp = xchp->next;
+
+				/* If so, show it after all. */
+				if (xchp != xch) {
+					xch = xchp;
+					goto pre_context_calculation;
+				}
 			}
 		}