Update from svn-1.8.14 to 1.9.2.

Formal release notes are available:

https://subversion.apache.org/docs/release-notes/1.9.html

Of particular note, the client checkout format has *not* changed so
upgrades should *not* be required.

When reading a repository (file:// or running as a local server), an
improved fsfs version 7 is available with significant performance
improvements. An optional upgrade is possible to use the new features.
Without the upgrade, this is fully read/write compatible with the
version 6 fsfs as in svn-1.8.

Relnotes: yes