This is pretty useful in avoiding races: I want to create a ref only if
it doesn't already exist.  I can't check first because of TOCTOU -- by
the time I finish the check, someone else might have already created
the ref.  And I can't take a lock because then I can't do the create,
since the create expects to take the lock.

The semantics are inspired by git update-ref, which allows an all-zero old
value to mean that the ref must not exist.

This requires adding a new symbolic ref to the testrepo fixture.
Some of the existing tests attempt to delete HEAD, expecting a different failure. Introduce and use a non-HEAD symbolic ref instead.
Adjust a few other tests as needed.

Fixes #5357

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.

Keep the reflog parameters as the last two, as they're the optional
parameters.

If the type of the on-disk reference has changed, the old value
comparison should fail.

Recognize when the reference has changed since we loaded it.

Bring the race detection goodness to symbolic references as well.