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.

The index iterator could previously only be created from a repo
object, but this allows creating an iterator from a `git_index`
object instead (while keeping, though renaming, the old function).

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.