Re: [PATCH v3 6/6] update-ref: add support for 'symref-update' command

> From: Karthik Nayak <karthik.188@gmail.com>
> 
> 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.