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.
Since the `apply` callback can defer, the `check` callback is not
necessary. Removing the `check` callback further makes the `payload`
unnecessary along with the `cleanup` callback.
Consumers can now register custom merged drivers with
`git_merge_driver_register`. This allows consumers to support the
merge drivers, as configured in `.gitattributes`. Consumers will be
asked to perform the file-level merge when a custom driver is
configured.
