unpack-trees: allow pruning with pathspec

Use the pathspec pruning of traverse_trees() from unpack_trees(). Again,
the unpack_trees() machinery is primarily meant for merging two (or more)
trees, and because a merge is a full tree operation, it didn't support any
pruning with pathspec, and this codepath probably should not be enabled
while running a merge, but the caller in diff-lib.c::diff_cache() should
be able to take advantage of it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -444,6 +444,7 @@ static int traverse_trees_recursive(int n, unsigned long dirmask,
 
 	newinfo = *info;
 	newinfo.prev = info;
+	newinfo.pathspec = info->pathspec;
 	newinfo.name = *p;
 	newinfo.pathlen += tree_entry_len(p->path, p->sha1) + 1;
 	newinfo.conflicts |= df_conflicts;
@@ -1040,6 +1041,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		info.fn = unpack_callback;
 		info.data = o;
 		info.show_all_errors = o->show_all_errors;
+		info.pathspec = o->pathspec;
 
 		if (o->prefix) {
 			/*

unpack-trees.h

@@ -51,6 +51,7 @@ struct unpack_trees_options {
 	const char *prefix;
 	int cache_bottom;
 	struct dir_struct *dir;
+	struct pathspec *pathspec;
 	merge_fn_t fn;
 	const char *msgs[NB_UNPACK_TREES_ERROR_TYPES];
 	/*