tree: widen struct tree.size and parse_tree_buffer() to size_t

Final piece of the tree topic. struct tree.size already receives
its values from size_t-shaped sources (odb_read_object() in
repo_parse_tree_gently() and in reflog.c::tree_is_complete()),
so on Windows it was already silently truncating anything past
4 GiB. Switch the field and parse_tree_buffer()'s size parameter
to size_t.

All readers feed tree->size into init_tree_desc(), which was
widened earlier in this topic; the existing parse_object_buffer()
caller in object.c keeps its unsigned long parameter, which
promotes cleanly.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

tree.c

@@ -172,7 +172,7 @@ struct tree *lookup_tree(struct repository *r, const struct object_id *oid)
 	return object_as_type(obj, OBJ_TREE, 0);
 }
 
-int parse_tree_buffer(struct tree *item, void *buffer, unsigned long size)
+int parse_tree_buffer(struct tree *item, void *buffer, size_t size)
 {
 	if (item->object.parsed)
 		return 0;

tree.h

@@ -10,14 +10,14 @@ struct strbuf;
 struct tree {
 	struct object object;
 	void *buffer;
-	unsigned long size;
+	size_t size;
 };
 
 extern const char *tree_type;
 
 struct tree *lookup_tree(struct repository *r, const struct object_id *oid);
 
-int parse_tree_buffer(struct tree *item, void *buffer, unsigned long size);
+int parse_tree_buffer(struct tree *item, void *buffer, size_t size);
 
 #define parse_tree_gently(t, q) repo_parse_tree_gently(the_repository, t, q)
 int repo_parse_tree_gently(struct repository *r, struct tree *item,