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.
Add tests for the case when there are no branches on the remote and when
HEAD is detached but has the id of a non-branch. In both of these cases,
we should return ENOTFOUND.
