refs: refuse to write pseudorefs

refs: refuse to write pseudorefs

Pseudorefs are not stored in the ref database as by definition, they
carry additional metadata that essentially makes them not a ref. As
such, writing pseudorefs via the ref backend does not make any sense
whatsoever as the ref backend wouldn't know how exactly to store the
data.

Restrict writing pseudorefs via the ref backend.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>