URL canonicalization with 1.6 clients and 1.7 servers

1.7 has stricter rules for canonical URLs (from RFC 3986) than 1.6:

 - no default port:

    "http://host/repo" not "http://host:80/repo"

 - no lowercase % encoding:

    "http://host/repo/%C3%A5" not "http://host/repo/%c3%a5"

 - no unnecessary % encoding:

    "http://host/repo/A" not "http://host/repo/%41"

All the above URLs can be used with a 1.7 client because the client
converts them to canonical form, but a 1.6 client will not do the
conversion and will pass the non-canonical form to the server. A 1.7
server will sometimes reject such URLs:

   $ svn-1.6 co http://localhost:80/repo
   svn: Path 'http://localhost:80/repo' is not canonicalized; there is a problem with the client.

although some commands work:

   $ svn-1.6 ls http://localhost:80/repo
   A

So this is a break in compatibility, which we justify by saying that the
URLs are not canonical. Could we do better? We could make the server
canonicalize the URL. That would probably allow the non-canonical URLs
to work, but might introduce problems like issue 3601:
http://subversion.tigris.org/issues/show_bug.cgi?id=3601

Should we attempt backwards compatibility or should we simply require
clients to upgrade or avoid the URLs?

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