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.
Already cherry-picked commits should not be re-included. If all changes
included in a commit exist in the upstream, then we should error with
GIT_EAPPLIED.
`git_rebase_next` will apply the next patch (or cherry-pick)
operation, leaving the results checked out in the index / working
directory so that consumers can resolve any conflicts, as appropriate.