dir_iterator: rewrite state machine model

Perform a rewrite of dir_iterator_advance(). dir_iterator has
ceased to rely on a combination of level.initialized and level.dir_state
state variables and now only tracks the state with level.dir_state,
which simplifies the iterator mechanism, makes the code easier to follow
and eases additions of new features to the iterator.

Make dir_iterator_begin() attempt to lstat() the path it receives, and
return NULL and an appropriate errno if it fails or if the passed path
was not a directory.

Create an option for the dir_iterator API to iterate over subdirectories
only after having iterated through their contents. This feature was
predicted, although not implemented by 0fe5043 ("dir_iterator: new API
for iterating over a directory tree", 2016-06-18). This is useful for
recursively removing a directory and calling rmdir() on a directory only
after all of its contents have been wiped.

Add an option for dir_iterator to also iterate over the initial
directory (the one passed to dir_iterator_begin()).

Add the "flags" parameter to dir_iterator_create, allowing for the
aforementioned new features to be enabled. The new default behavior
(i.e. flags set to 0) does not iterate over directories. Flag
DIR_ITERATOR_PRE_ORDER_TRAVERSAL iterates over a directory before doing
so over its contents. DIR_ITERATOR_POST_ORDER_TRAVERSAL iterates over a
directory after doing so over its contents. DIR_ITERATOR_LIST_ROOT_DIR
iterates over the initial directory. These flags do not conflict with
each other and may be used simultaneously.

Amend a call to dir_iterator_begin() in refs/files-backend.c to pass
the flags parameter introduced, as well as handle the case in which it
fails to open the directory.

Improve t/t0066-dir-iterator.sh and t/helper/test-dir-iterator.c to
test "post-order" and "iterate-over-root" modes.

Michael Haggerty contributed with the design of the new
dir_iterator_advance() implementation, the code for
t/helper/test-dir-iterator's option parser and numerous reviews that
gradually shaped this code to its current form.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Daniel Ferreira <bnmvco@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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