The xdl_prepare_env() function may initialise an xdlclassifier_t
data structure via xdl_init_classifier(), which allocates memory
to several fields, for example 'rchash', 'rcrecs' and 'ncha'.
If this function later exits due to the failure of xdl_optimize_ctxs(),
then this xdlclassifier_t structure, and the memory allocated to it,
is not cleaned up.

In order to fix the memory leak, insert a call to xdl_free_classifier()
before returning.

This patch was originally written by Ramsay Jones (see commit
87f16258367a3b9a62663b11f898a4a6f3c19d31 in git.git).

Commit 307ab20b3 ("xdiff: PATIENCE/HISTOGRAM are not independent option
bits", 19-02-2012) introduced the XDF_DIFF_ALG() macro to access the
flag bits used to represent the diff algorithm requested. In addition,
code which had used explicit manipulation of the flag bits was changed
to use the macros.

However, one example of direct manipulation remains. Update this code to
use the XDF_DIFF_ALG() macro.

This patch was originally written by Ramsay Jones (see commit
5cd6978a9cfef58de061a9525f3678ade479564d in git.git).

config: don't write duplicate section

config: don't special-case multivars that don't exist yet

If we hit the EOF while trying to write a new value, it may be that
we're already in the section that we were looking for.  If so, do not
write a (duplicate) section header, just write the value.

We should notice that we are in the correct section to add. This is a
cosmetic bug, since replacing any of these settings does work.

CMakeLists: Further improve the error messages regarding CMAKE_SIZEOF_VOID_P

Some FANTASTIC iterator refactoring

Remove some unused functions, refactor some ugliness.

(It's slow!)

Since the three iterators implement `advance_over` differently,
mandate it and implement each.

Iterator tests were split over repo::iterator and diff::iterator,
with duplication between the two.  Move them to iterator::index,
iterator::tree, and iterator::workdir.

Buffer sideband packet data

When a directory is removed out from underneath us, stop trying to
manipulate it.

Now that iterators do not return `GIT_ENOTFOUND` when advancing
into an empty directory, we do not need a special `advance_into_or_over`
function.

Prior iterator implementations returned `GIT_ENOTFOUND` when
trying to advance into empty directories.  Ensure that we no longer
do that and simply handle them gracefully.

tree_iterator was only working properly for a pathlist containing
file paths. In case of directory paths, it didn't match children
which contradicts GIT_DIFF_DISABLE_PATHSPEC_MATCH and
is different from index_iterator and fs_iterator.

As a consequence head-to-index status reporting for a specific
directory did not work properly -- all files have been reported
as added.

Include additional tests.

`git_iterator_advance_over` is a gnarly bit of code with no actual
tests.

In the workdir iterator we do some tricky things to step down into
directories to look for things that are in our pathlist.  Make sure
that we don't confuse between folders that we're definitely going to
return everything in and folders that we're only stepping down into
to keep looking for matches.

Ensure that when specifying start/end paths, or pathlists, that we
deal correctly with submodules.

Expand the workdir tests to validate the paths in case sensitive
and insensitive tests.

Ensure that we have hit the end of iteration; previously we tested
that we saw all the values that we expected to see.  We did not
then ensure that we were at the end of the iteration (and that there
were subsequently values in the iteration that we did *not* expect.)

Drop some of the layers of indirection between the workdir and the
filesystem iterators.  This makes the code a little bit easier to
follow, and reduces the number of unnecessary allocations a bit as
well.  (Prior to this, when we filter entries, we would allocate them,
filter them and then free them; now we do the filtering before
allocation.)

Also, rename `git_iterator_advance_over_with_status` to just
`git_iterator_advance_over`.  Mostly because it's a fucking long-ass
function name otherwise.

Do not abort iteration in the middle when encountering an unreadable
directory.  Instead, skip it, as if it didn't exist.

Many code paths in checkout need the final, full on-disk path of the
file they're writing.  (No surprise).  However, they all munge the
`data->path` buffer themselves to get there.  Provide a nice helper
method for them.

Plus, drop the use `git_iterator_current_workdir_path` which does the
same thing but different.  Checkout is the only caller of this silly
function, which lets us remove it.