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.

Refactored the tree iterator to never recurse; simply process the
next entry in order in `advance`.  Additionally, reduce the number of
allocations and sorting as much as possible to provide a ~30% speedup
on case-sensitive iteration.  (The gains for case-insensitive iteration
are less majestic.)