path-walk: allow consumer to specify object types

We add the ability to filter the object types in the path-walk API so
the callback function is called fewer times.

This adds the ability to ask for the commits in a list, as well. We
re-use the empty string for this set of objects because these are passed
directly to the callback function instead of being part of the
'path_stack'.

Future changes will add the ability to visit annotated tags.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path-walk.h

@@ -31,9 +31,21 @@ struct path_walk_info {
 	 */
 	path_fn path_fn;
 	void *path_fn_data;
+
+	/**
+	 * Initialize which object types the path_fn should be called on. This
+	 * could also limit the walk to skip blobs if not set.
+	 */
+	int commits;
+	int trees;
+	int blobs;
 };
 
-#define PATH_WALK_INFO_INIT { 0 }
+#define PATH_WALK_INFO_INIT {   \
+	.blobs = 1,		\
+	.trees = 1,		\
+	.commits = 1,		\
+}
 
 void path_walk_info_init(struct path_walk_info *info);
 void path_walk_info_clear(struct path_walk_info *info);