Fix sparse warnings.

Mainly making a lot of local functions and variables be marked "static",
but there was a "zero as NULL" warning in there too.

ls-tree.c

@@ -79,7 +79,7 @@ static struct tree_entry_list *find_entry(const char *path)
 		slash = strchr(path, '/');
 		if (!slash) {
 			len = strlen(path);
-			next = 0;
+			next = NULL;
 		}
 		else {
 			next = slash + 1;