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

[Josh Triplett <josh@joshtriplett.org>]

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