tree_entry_interesting(): support depth limit

This is needed to replace pathspec_matches() in builtin/grep.c.

max_depth == -1 means infinite depth. Depth limit is only effective
when pathspec.recursive == 1. When pathspec.recursive == 0, the
behavior depends on match functions: non-recursive for
tree_entry_interesting() and recursive for match_pathspec{,_depth}

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-diff.c

@@ -146,6 +146,10 @@ int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
 	int all_t1_interesting = 0;
 	int all_t2_interesting = 0;
 
+	/* Enable recursion indefinitely */
+	opt->pathspec.recursive = DIFF_OPT_TST(opt, RECURSIVE);
+	opt->pathspec.max_depth = -1;
+
 	strbuf_init(&base, PATH_MAX);
 	strbuf_add(&base, base_str, baselen);