dir.c: provide clear_directory() for reclaiming dir_struct memory

By the end of a directory traversal, a dir_struct instance will
typically contains pointers to various data structures on the heap.
clear_directory() provides a convenient way to reclaim that memory.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-directory-listing.txt

@@ -81,4 +81,6 @@ marked. If you to exclude files, make sure you have loaded index first.
 
 * Use `dir.entries[]`.
 
+* Call `free_directory()` when none of the contained elements are no longer in use.
+
 (JC)

dir.c

@@ -1557,3 +1557,33 @@ void free_pathspec(struct pathspec *pathspec)
 	free(pathspec->items);
 	pathspec->items = NULL;
 }
+
+/*
+ * Frees memory within dir which was allocated for exclude lists and
+ * the exclude_stack.  Does not free dir itself.
+ */
+void clear_directory(struct dir_struct *dir)
+{
+	int i, j;
+	struct exclude_list_group *group;
+	struct exclude_list *el;
+	struct exclude_stack *stk;
+
+	for (i = EXC_CMDL; i <= EXC_FILE; i++) {
+		group = &dir->exclude_list_group[i];
+		for (j = 0; j < group->nr; j++) {
+			el = &group->el[j];
+			if (i == EXC_DIRS)
+				free((char *)el->src);
+			clear_exclude_list(el);
+		}
+		free(group->el);
+	}
+
+	stk = dir->exclude_stack;
+	while (stk) {
+		struct exclude_stack *prev = stk->prev;
+		free(stk);
+		stk = prev;
+	}
+}

dir.h

@@ -169,6 +169,7 @@ extern void parse_exclude_pattern(const char **string, int *patternlen, int *fla
 extern void add_exclude(const char *string, const char *base,
 			int baselen, struct exclude_list *el, int srcpos);
 extern void clear_exclude_list(struct exclude_list *el);
+extern void clear_directory(struct dir_struct *dir);
 extern int file_exists(const char *);
 
 extern int is_inside_dir(const char *dir);