When proxying the SVN protocol, don't mutate URIs in protocol frames if we're an intracluster proxy

Summary:
Ref T10809. Currently, both the proxy and target may mutate URIs (rewriting "svn+ssh://x/diffusion/Y/" to a path on disk).

I believe this previously worked by fate/chance/luck since both URI variants contain the repository information, but the algorithms were tightened up recently with callsign removal.

Stop rewriting them if we're the intracluster proxy -- they only need to be rewritten on the target host.

Test Plan:

• Checked out a proxied SVN repository, with and without a callsign.
• Checked out an unproxied SVN repository, with and without a callsign.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10809

Differential Revision: https://secure.phabricator.com/D15712