refs/reftable: perform explicit D/F check when writing symrefs

refs/reftable: perform explicit D/F check when writing symrefs

We already perform explicit D/F checks in all reftable callbacks which
write refs, except when writing symrefs. For one this leads to an error
message which isn't perfectly actionable because we only tell the user
that there was a D/F conflict, but not which refs conflicted with each
other. But second, once all ref updating callbacks explicitly check for
D/F conflicts, we can disable the D/F checks in the reftable library
itself and thus avoid some duplicated efforts.

Refactor the code that writes symref tables to explicitly call into
`refs_verify_refname_available()` when writing symrefs.

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