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.
When we update the current branch, we must also append to HEAD's reflog
to keep them in sync.
This is a bit of a hack, but as git.git says, it covers 100% of
default cases.
