Upgrade svn 1.7 repositories automatically.

Subversion 1.7 requires a manual upgrade of the working copy in order to
do anything (even to run svn status). Luckily there's an error message
ID for this case so it's not too hard to grep for situations where this
is necessary.

One problematic element is that since even 'svn info' doesn't work in
this situation, --pretend mode doesn't work either for svn modules. For
now I just throw an exception (after the check) when in pretend mode so
avoid follow-on useless error messages.

It was interesting how I still had the old 1.6.17 for so long -- turns
out I had forgot to rename my subversion selector asking for unstable
versions in Gentoo (at some point their package name changed from
dev-util/subversion to dev-vcs/subversion, which I didn't keep up with).

Thanks Will Stephenson for the bug report.

BUG:304415