add -p: adjust offsets of subsequent hunks when one is skipped

Since commit 8cbd431082 ("git-add--interactive: replace hunk
recounting with apply --recount", 2008-7-2) if a hunk is skipped then
we rely on the context lines to apply subsequent hunks in the right
place. While this works most of the time it is possible for hunks to
end up being applied in the wrong place. To fix this adjust the offset
of subsequent hunks to correct for any change in the number of
insertions or deletions due to the skipped hunk. The change in offset
due to edited hunks that have the number of insertions or deletions
changed is ignored here, it will be fixed in the next commit.

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

git-add--interactive.perl

@@ -926,14 +926,25 @@ sub coalesce_overlapping_hunks {
 	my @out = ();
 
 	my ($last_o_ctx, $last_was_dirty);
+	my $ofs_delta = 0;
 
-	for (grep { $_->{USE} } @in) {
+	for (@in) {
 		if ($_->{TYPE} ne 'hunk') {
 			push @out, $_;
 			next;
 		}
 		my $text = $_->{TEXT};
-		my ($o_ofs) = parse_hunk_header($text->[0]);
+		my ($o_ofs, $o_cnt, $n_ofs, $n_cnt) =
+						parse_hunk_header($text->[0]);
+		unless ($_->{USE}) {
+			$ofs_delta += $o_cnt - $n_cnt;
+			next;
+		}
+		if ($ofs_delta) {
+			$n_ofs += $ofs_delta;
+			$_->{TEXT}->[0] = format_hunk_header($o_ofs, $o_cnt,
+							     $n_ofs, $n_cnt);
+		}
 		if (defined $last_o_ctx &&
 		    $o_ofs <= $last_o_ctx &&
 		    !$_->{DIRTY} &&

t/t3701-add-interactive.sh

@@ -496,7 +496,7 @@ test_expect_success 'set up pathological context' '
 	test_write_lines +b " a" >patch
 '
 
-test_expect_failure 'add -p works with pathological context lines' '
+test_expect_success 'add -p works with pathological context lines' '
 	git reset &&
 	printf "%s\n" n y |
 	git add -p &&