t3701: add failing test for pathological context lines

When a hunk is skipped by add -i the offsets of subsequent hunks are
not adjusted to account for any missing insertions due to the skipped
hunk. Most of the time this does not matter as apply uses the context
lines to apply the subsequent hunks in the correct place, however in
pathological cases the context lines will match at the now incorrect
offset and the hunk will be applied in the wrong place. The offsets of
hunks following an edited hunk that has had the number of insertions
or deletions changed also need to be updated in the same way. Add
failing tests to demonstrate this.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3701-add-interactive.sh

@@ -483,4 +483,34 @@ test_expect_success 'add -p works even with color.ui=always' '
 	test_cmp expect actual
 '
 
+test_expect_success 'set up pathological context' '
+	git reset --hard &&
+	test_write_lines a a a a a a a a a a a >a &&
+	git add a &&
+	git commit -m a &&
+	test_write_lines c b a a a a a a a b a a a a >a &&
+	test_write_lines     a a a a a a a b a a a a >expected-1 &&
+	test_write_lines   b a a a a a a a b a a a a >expected-2 &&
+	# check editing can cope with missing header and deleted context lines
+	# as well as changes to other lines
+	test_write_lines +b " a" >patch
+'
+
+test_expect_failure 'add -p works with pathological context lines' '
+	git reset &&
+	printf "%s\n" n y |
+	git add -p &&
+	git cat-file blob :a >actual &&
+	test_cmp expected-1 actual
+'
+
+test_expect_failure 'add -p patch editing works with pathological context lines' '
+	git reset &&
+	# n q q below is in case edit fails
+	printf "%s\n" e y    n q q |
+	git add -p &&
+	git cat-file blob :a >actual &&
+	test_cmp expected-2 actual
+'
+
 test_done