dir-iterator: refactor state machine model

dir_iterator_advance() is a large function with two nested loops. Let's
improve its readability factoring out three functions and simplifying
its mechanics. The refactored model will no longer depend on
level.initialized and level.dir_state to keep track of the iteration
state and will perform on a single loop.

Also, dir_iterator_begin() currently does not check if the given string
represents a valid directory path. Since the refactored model will have
to stat() the given path at initialization, let's also check for this
kind of error and make dir_iterator_begin() return NULL, on failures,
with errno appropriately set. And add tests for this new behavior.

Improve documentation at dir-iteration.h and code comments at
dir-iterator.c to reflect the changes and eliminate possible
ambiguities.

Finally, adjust refs/files-backend.c to check for now possible
dir_iterator_begin() failures.

Original-patch-by: Daniel Ferreira <bnmvco@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-dir-iterator.c

@@ -17,6 +17,11 @@ int cmd__dir_iterator(int argc, const char **argv)
 
 	diter = dir_iterator_begin(path.buf);
 
+	if (!diter) {
+		printf("dir_iterator_begin failure: %d\n", errno);
+		exit(EXIT_FAILURE);
+	}
+
 	while (dir_iterator_advance(diter) == ITER_OK) {
 		if (S_ISDIR(diter->st.st_mode))
 			printf("[d] ");