Pseudorefs should not be updated through the ref transaction
API, because alternate ref backends still need to store pseudorefs
in GIT_DIR (instead of wherever they store refs). Instead,
change update_ref and delete_ref to call pseudoref-specific
functions.
Signed-off-by: David Turner <dturner@twopensource.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>