completion: treat dangling symrefs as existing pseudorefs

completion: treat dangling symrefs as existing pseudorefs

The `__git_pseudoref_exists ()` helper function back to git-rev-parse(1)
in case the reftable backend is in use. This is not in the same spirit
as the simple existence check that the "files" backend does though,
because there we only check for the pseudo-ref to exist with `test -f`.
With git-rev-parse(1) we not only check for existence, but also verify
that the pseudo-ref resolves to an object, which may not be the case
when the pseudo-ref points to an unborn branch.

Fix this issue by using `git show-ref --exists` instead. Note that we do
not have to silence stdout anymore as git-show-ref(1) will not print
anything.

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