refs: support symrefs in 'reference-transaction' hook

The 'reference-transaction' hook runs whenever a reference update is
made to the system. In a previous commit, we added the `old_target` and
`new_target` fields to the `reference_transaction_update()`. In
following commits we'll also add the code to handle symref's in the
reference backends.

Support symrefs also in the 'reference-transaction' hook, by modifying
the current format:
    <old-oid> SP <new-oid> SP <ref-name> LF
to be be:
    <old-value> SP <new-value> SP <ref-name> LF
where for regular refs the output would not change and remain the same.
But when either 'old-value' or 'new-value' is a symref, we print the ref
as 'ref:<ref-target>'.

This does break backward compatibility, but the 'reference-transaction'
hook's documentation always stated that support for symbolic references
may be added in the future.

We do not add any tests in this commit since there is no git command
which activates this flow, in an upcoming commit, we'll start using
transaction based symref updates as the default, we'll add tests there
for the hook too.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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