upload-pack/ls-remote: showing non-HEAD symbolic refs?

Commit 5e7dcad771cb873e278a0571b46910d7c32e2f6c in September 2013 added support to upload-pack to show the symbolic target of non-HEAD symbolic refs. However, commit d007dbf7d6d647dbcf0f357545f43f36dec46f3b in November 2013 reverted that, because it used a capability to transmit the information, and capabilities have a limited size (limited by the pkt-line format which can't send lines longer than 64k) and can't transmit an arbitrary number of symrefs.

(Incidentally, couldn't the same problem occur if the HEAD points to a long enough path to exceed 64k? Unlikely to arise in practice, but theoretically possible for a constructed repository. Not a major problem at the moment, since send-pack doesn't seem to support *sending* symbolic refs, but it would become a problem given any mechanism to send symbolic refs to the server.)

I'd like to be able to see the targets of non-HEAD symbolic refs for a repository (symbolic refs under refs/). I'm interested in extending upload-pack to expose those somehow. What seems like a sensible format to do so?

Would it make sense to advertise a new capability for symbolic ref targets, which would allow the client to send back a dedicated request for the targets of all symrefs?

- Josh Triplett