Log Message:
Update to 1.6.21

Changelog:
Version 1.6.21
(04 Apr 2013, from /branches/1.6.x)
http://svn.apache.org/repos/asf/subversion/tags/1.6.21

 User-visible changes
  - Server-side bugfixes:
    * mod_dav_svn will omit some property values for activity urls (r1453780)
    * improve memory usage when committing properties in mod_dav_svn (r1443929)
    * fix mod_dav_svn runs pre-revprop-change twice (issue #3085)
    * fixed: post-revprop-change errors cancel commit (issue #2990)
    * improved logic in mod_dav_svn's implementation of lock. (r1455352)

 Developer-visible changes:
  - General:
    * fix a compatibility issue with g++ 4.7 (r1345740)

This release addesses four security issues:
    CVE-2013-1845: mod_dav_svn excessive memory usage from property changes
    CVE-2013-1846: mod_dav_svn crashes on LOCK requests against activity URLs
    CVE-2013-1847: mod_dav_svn crashes on LOCK requests against
non-existant URLs
    CVE-2013-1849: mod_dav_svn crashes on PROPFIND requests against
activity URLs