`git_rebase_init` and `git_rebase_open` should take a
`git_rebase_options` and use it for future rebase operations on
that `rebase` object.

In `git_rebase_operation_current()`, indicate when a rebase has not
started (with `GIT_REBASE_NO_OPERATION`) rather than conflating that
with the first operation being in-progress.

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.

Don't require the branch to rebase, if given `NULL`, simply look up
`HEAD`.

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