[PATCH] Fix svnversion message as follow-up to r922176

Hi!

The 1.6 svnversion message was "'path' not versioned, and not exported".
But on trunk more than one message has been changed. My first thought
was that we should be backward compat in our output but if changes of
those messages are ok I'm supplying one.

In case we will use new messages, the help text must be updated. It
talks of 'exported' but those are not used in the new messages.

[[[
After the changes in r922176, versioned but not yet committed files were
not properly detected. Fixed now!

* subversion/svnversion/main.c
  (main): Check for invalid rev nr for files and dirs.

* subversion/tests/cmdline/svnversion_tests.py
  (structural_changes): New.
  (tests_list): Add new test.
]]]