update-ref: add support for 'symref-update' command

update-ref: add support for 'symref-update' command

Add 'symref-update' command to the '--stdin' mode of 'git-update-ref' to
allow updates of symbolic refs. The 'symref-update' command takes in a
<new-target>, which the <ref> will be updated to. If the <ref> doesn't
exist it will be created.

It also optionally takes either an `ref <old-target>` or `oid
<old-oid>`. If the <old-target> is provided, it checks to see if the
<ref> targets the <old-target> before the update. If <old-oid> is provided
it checks <ref> to ensure that it is a regular ref and <old-oid> is the
OID before the update. This by extension also means that this when a
zero <old-oid> is provided, it ensures that the ref didn't exist before.

The divergence in syntax from the regular `update` command is because if
we don't use a `(ref | oid)` prefix for the old_value, then there is
ambiguity around if the value provided should be treated as an oid or a
reference. This is more so the reason, because we allow anything
committish to be provided as an oid. While 'symref-verify' and
'symref-delete' also take in `<old-target>` we do not have this
divergence there as those commands only work with symrefs. Whereas
'symref-update' also works with regular refs and allows users to convert
regular refs to symrefs.

The command allows users to perform symbolic ref updates within a
transaction. This provides atomicity and allows users to perform a set
of operations together.

This command supports deref mode, to ensure that we can update
dereferenced regular refs to symrefs.

Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>