lock_ref_for_update(): avoid a symref resolution

lock_ref_for_update(): avoid a symref resolution

If we're overwriting a symref with a SHA-1, we need to resolve the value
of the symref (1) to check against update->old_sha1 and (2) to write to
its reflog. However, we've already read the symref itself and know its
referent. So there is no need to read the symref's value through the
symref; we can read the referent directly.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>