If given a NULL path, write to the object path of the repository.

Add tests for the new behavior.

Docurium seems to choke on this header because it can't see both
git_indexer_progress & git_indexer_progress_cb, let's add the include.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

After 1cd65991, we were passing a pointer to an `unsigned long` to
a function that now expected a pointer to a `size_t`.  These types
differ on 64-bit Windows, which means that we trash the stack.

Use `size_t`s in the packbuilder to avoid this.

This function recursively inserts the given object and any referenced
ones. It can be thought of as a more general version of the functions to
insert a commit or tree.

Most use-cases for the object packer communicate in terms of commits
which each side has. We already have an object to specify this
relationship between commits, namely git_revwalk.

By knowing which commits we want to pack and which the other side
already has, we can perform similar optimisations to git, by marking
each tree as interesting or uninteresting only once, and not sending
those trees which we know the other side has.