When diff encounters an untracked directory, there was a shortcut
that it took which is not compatible with core git.  This makes
the default behavior no longer take that shortcut and instead look
inside the untracked directory to see if there are any untracked
files within it.  If there are not, then the directory is treated
as an ignore directory instead of an untracked directory.  This
has implications for the git_status APIs.

This fixes some missed places where we can apply const-ness to
various public APIs.

There are still some index and tree APIs that cannot take const
pointers because we sort our `git_vectors` lazily and so we can't
reliably bsearch the index and tree content without applying a
`git_vector_sort()` first.

This also fixes some missed places where size_t can be used and
where const can be applied to a couple internal functions.

Initial implementation of diff rename detection

This improves the naming for the rename related functionality
moving it to be called `git_diff_find_similar()` and renaming
all the associated constants, etc. to make more sense.

I also moved the new code (plus the existing `git_diff_merge`)
into a new file `diff_tform.c` where I can put new functions
related to manipulating git diff lists.

This also updates the implementation significantly from the
last revision fixing some ordering issues (where break-rewrite
needs to be handled prior to copy and rename detection) and
improving config option handling.