subrepo/svn: make rev number retrieval compatible with svn 1.5 (issue2968)

ae2664ee0223 introduced "svn info TARGET@REV" to determine if a certain
path exists in the specified revision, but in svn 1.5 the error message
"Not a valid URL" yields exit code 0 so the error is not caught.
Use "svn list TARGET@REV" instead which works with svn 1.5 and is even
faster in some situations.

mercurial/subrepo.py

@@ -730,7 +730,7 b' class svnsubrepo(abstractsubrepo):'
             # URL exists at lastrev.  Test it and fallback to rev it
             # is not there.
             try:
-                self._svncommand(['info', '%s@%s' % (self._state[0], lastrev)])
+                self._svncommand(['list', '%s@%s' % (self._state[0], lastrev)])
                 return lastrev
             except error.Abort:
                 pass