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.

We have no need to take a non-const reference. This does involve some other work
to make sure we don't mix const and non-const variables, but by splitting what
we want each variable to do we can also simplify the logic for when we do want
to free a new reference we might have allocated.

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).

References passed to the callback function of `git_reference_foreach`
are expected to be owned by the callback. As such, they are never being
freed by `git_reference_foreach`, but will have to be freed by the
caller. This small detail is never mentioned in the function's
documentation, though, making it easy to get wrong. Document this to
make it discoverable.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

This makes them show up in the reference, even if the text itself isn't
the most descriptive.

These have been found with

    grep -Przon '\n\ntypedef struct.*?\{' -- include
    grep -Przon '\n\ntypedef enum.*?\{' -- include

If it's not documented, it doesn't show up in the docs (and we really
should document, anyway).