This adds an option which will check if a diff is applicable without
actually applying it; equivalent to git apply --check.

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.

Introduce a callback to patch application that allows consumers to
cancel hunk application.