config_file: add list holding config entries in order of appearance

Right now, we only keep all configuration entries in a string map. This
is required to efficiently access configuration entries by keys. It has
the disadvantage of not being able to iterate through configuration
entries in the order they were read, though. Instead, we only iterate
through entries in a seemingly random order.

Extend `diskfile_entries` by another list holding configuration entries.
Like this, we maintain all entries in two data structures and can use
the required one based on the current use case.

src/config_file.c

@@ -40,6 +40,7 @@ typedef struct git_config_file_iter {
 typedef struct {
 	git_atomic refcount;
 	git_strmap *map;
+	config_entry_list *list;
 } diskfile_entries;
 
 typedef struct {
@@ -129,6 +130,19 @@ int git_config_file_normalize_section(char *start, char *end)
 	return 0;
 }
 
+static void config_entry_list_append(config_entry_list **list, config_entry_list *entry)
+{
+	config_entry_list *head = *list;
+
+	if (head) {
+		while (head->next != NULL)
+			head = head->next;
+		head->next = entry;
+	} else {
+		*list = entry;
+	}
+}
+
 /* Add or append the new config option */
 static int diskfile_entries_append(diskfile_entries *entries, git_config_entry *entry)
 {
@@ -143,23 +157,25 @@ static int diskfile_entries_append(diskfile_entries *entries, git_config_entry *
 	pos = git_strmap_lookup_index(entries->map, entry->name);
 	if (!git_strmap_valid_index(entries->map, pos)) {
 		git_strmap_insert(entries->map, entry->name, var, &error);
+
+		if (error > 0)
+			error = 0;
 	} else {
 		existing = git_strmap_value_at(entries->map, pos);
-		while (existing->next != NULL) {
-			existing = existing->next;
-		}
-		existing->next = var;
+		config_entry_list_append(&existing, var);
 	}
 
-	if (error > 0)
-		error = 0;
+	var = git__calloc(1, sizeof(config_entry_list));
+	GITERR_CHECK_ALLOC(var);
+	var->entry = entry;
+	config_entry_list_append(&entries->list, var);
 
 	return error;
 }
 
 static void diskfile_entries_free(diskfile_entries *entries)
 {
-	config_entry_list *list = NULL;
+	config_entry_list *list = NULL, *next;
 
 	if (!entries)
 		return;
@@ -169,6 +185,14 @@ static void diskfile_entries_free(diskfile_entries *entries)
 
 	git_strmap_foreach_value(entries->map, list, config_entry_list_free(list));
 	git_strmap_free(entries->map);
+
+	list = entries->list;
+	while (list != NULL) {
+		next = list->next;
+		git__free(list);
+		list = next;
+	}
+
 	git__free(entries);
 }