Stop returning a void for functions, future-proofing them to allow them
to fail.

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 git_revwalk objects are encouraged to be reused, a public
interface for changing hide_cb is desirable.

Show more directly what the sorting modes correspond to in git's `rev-list` as
that's the reference implementation for what the possible sorting orders are.

It changed from implementation-defined to git's default sorting, as there are
systems (e.g. rebase) which depend on this order. Also specify more explicitly
how you can get git's "date-order".

As it seems it's not quite clear what it means to push a commit, try to
be more explicit about adding a new root and that we may not see this
commit if it is hidden.