refs: handle zero oid for pseudorefs

refs: handle zero oid for pseudorefs

According to the documentation, it is possible to "specify 40 '0' or an
empty string as <oldvalue> to make sure that the ref you are creating
does not exist." But in the code for pseudorefs, we do not implement
this, as demonstrated by the failing tests added in the previous commit.
If we fail to read the old ref, we immediately die. But a failure to
read would actually be a good thing if we have been given the zero oid.

With the zero oid, allow -- and even require -- the ref-reading to fail.
This implements the "make sure that the ref ... does not exist" part of
the documentation and fixes both failing tests from the previous commit.

Since we have a `strbuf err` for collecting errors, let's use it and
signal an error to the caller instead of dying hard.

Reported-by: Rafael Ascensão <rafa.almas@gmail.com>
Helped-by: Rafael Ascensão <rafa.almas@gmail.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>