[RFC] Include the svn version number in the "Working copy is too old" error message

Currently (at r1050442), running a 1.7-dev svn on a 1.6 WC gives:

  $ svn st
  svn: Please see the 'svn upgrade' command
  svn: Working copy format of '/home/julianfoad/src/subversion-n-gamma'
is too old (10)

I would like to see a more user-friendly message, giving the version
number instead of (or as well as) the WC format number, e.g.:

  $ svn st
  svn: Please see the 'svn upgrade' command
  svn: Working copy '/home/julianfoad/src/subversion-n-gamma' is too old
(format 10, created by svn 1.6)

Agreed so far?

The simple way is to embed a (format => version) conversion table in
libsvn_wc where this message is generated. Patch attached.

The more "proper" layered way is for the the client to have that
knowledge and do that conversion. That requires somehow passing the
format number of that WC path to the client. (Extracting that info from
the error message is the wrong way, of course - and not portable to
other locales.) Lacking parameterized error objects, I can't think of a
practical way to achieve this.

I think the simple patch (or something very like it) should be
acceptable. What do you think? If not, any bright ideas how to do it
better?

- Julian