diff: fix leaking orderfile option

The `orderfile` diff option is being assigned via `OPT_FILENAME()`,
which assigns an allocated string to the variable. We never free it
though, causing a memory leak.

Change the type of the string to `char *` and free it to plug the leak.
This also requires us to use `xstrdup()` to assign the global config to
it in case it is set.

This leak is being hit in t7621, but plugging it alone does not make the
test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

combine-diff.c

@@ -1393,9 +1393,8 @@ static struct combine_diff_path *find_paths_generic(const struct object_id *oid,
 {
 	struct combine_diff_path *paths = NULL;
 	int i, num_parent = parents->nr;
-
 	int output_format = opt->output_format;
-	const char *orderfile = opt->orderfile;
+	char *orderfile = opt->orderfile;
 
 	opt->output_format = DIFF_FORMAT_NO_OUTPUT;
 	/* tell diff_tree to emit paths in sorted (=tree) order */

diff.c

@@ -441,8 +441,10 @@ int git_diff_ui_config(const char *var, const char *value,
 	}
 	if (!strcmp(var, "diff.wordregex"))
 		return git_config_string(&diff_word_regex_cfg, var, value);
-	if (!strcmp(var, "diff.orderfile"))
+	if (!strcmp(var, "diff.orderfile")) {
+		FREE_AND_NULL(diff_order_file_cfg);
 		return git_config_pathname(&diff_order_file_cfg, var, value);
+	}
 
 	if (!strcmp(var, "diff.ignoresubmodules")) {
 		if (!value)
@@ -4775,7 +4777,7 @@ void repo_diff_setup(struct repository *r, struct diff_options *options)
 	if (diff_indent_heuristic)
 		DIFF_XDL_SET(options, INDENT_HEURISTIC);
 
-	options->orderfile = diff_order_file_cfg;
+	options->orderfile = xstrdup_or_null(diff_order_file_cfg);
 
 	if (!options->flags.ignore_submodule_set)
 		options->flags.ignore_untracked_in_submodules = 1;
@@ -6727,6 +6729,7 @@ void diff_free(struct diff_options *options)
 		FREE_AND_NULL(options->objfind);
 	}
 
+	FREE_AND_NULL(options->orderfile);
 	for (size_t i = 0; i < options->anchors_nr; i++)
 		free(options->anchors[i]);
 	FREE_AND_NULL(options->anchors);

diff.h

@@ -235,7 +235,7 @@ enum diff_submodule_format {
  * diffcore library with.
  */
 struct diff_options {
-	const char *orderfile;
+	char *orderfile;
 
 	/*
 	 * "--rotate-to=<file>" would start showing at <file> and when