fix segfault with git log -c --follow

In diff_tree_combined we make a copy of diffopts. In
try_to_follow_renames, called via diff_tree_sha1, we free and
re-initialize diffopts->pathspec->items. Since we did not make a deep
copy of diffopts in diff_tree_combined, the original diffopts does not
get the update. By the time we return from diff_tree_combined,
rev->diffopt->pathspec->items points to an invalid memory address. We
get a segfault next time we try to access that pathspec.

Instead, along with the copy of diffopts, make a copy pathspec->items as
well.

We would also have to make a copy of pathspec->raw to keep it consistent
with pathspec->items, but nobody seems to rely on that.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1120,6 +1120,7 @@ void diff_tree_combined(const unsigned char *sha1,
 	int i, num_paths, needsep, show_log_first, num_parent = parents->nr;
 
 	diffopts = *opt;
+	diff_tree_setup_paths(diffopts.pathspec.raw, &diffopts);
 	diffopts.output_format = DIFF_FORMAT_NO_OUTPUT;
 	DIFF_OPT_SET(&diffopts, RECURSIVE);
 	DIFF_OPT_CLR(&diffopts, ALLOW_EXTERNAL);
@@ -1187,6 +1188,8 @@ void diff_tree_combined(const unsigned char *sha1,
 		paths = paths->next;
 		free(tmp);
 	}
+
+	diff_tree_release_paths(&diffopts);
 }
 
 void diff_tree_combined_merge(const struct commit *commit, int dense,

t/t4202-log.sh

@@ -542,6 +542,20 @@ test_expect_success 'show added path under "--follow -M"' '
 	)
 '
 
+test_expect_success 'git log -c --follow' '
+	test_create_repo follow-c &&
+	(
+		cd follow-c &&
+		test_commit initial file original &&
+		git rm file &&
+		test_commit rename file2 original &&
+		git reset --hard initial &&
+		test_commit modify file foo &&
+		git merge -m merge rename &&
+		git log -c --follow file2
+	)
+'
+
 cat >expect <<\EOF
 *   commit COMMIT_OBJECT_NAME
 |\  Merge: MERGE_PARENTS