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.

git_object_t is the future; update the public API to use it.  This will
also ensure that we can build our tests which make use of the old API
without modification (and without compiler warnings).

Several functions to retrieve variables from a repository only return
immutable values, which allows us to actually constify the passed-in
repository parameter. Do so to help a later patch, which will only have
access to a constant repository.

Disambiguate error values: return `GIT_ENOTFOUND` when the item cannot
exist in the repository (perhaps because the repository is inmemory or
otherwise not backed by a filesystem), return `-1` when there is a hard
failure.

Implement `git_repository_head_for_worktree` and
`git_repository_head_detached_for_worktree` for directly accessing a
worktree's HEAD without opening it as a `git_repository` first.

Add function `git_repository_open_from_worktree`, which allows to open a
`git_worktree` as repository.