Update Subversion to 1.9.4

Changelog:
Version 1.9.4
(28 Apr 2016, from /branches/1.9.x)
http://svn.apache.org/repos/asf/subversion/tags/1.9.4

User-visible changes:
  - Client-side bugfixes:
    * diff: support '--summarize --ignore-properties' (part of issue #4567)
    * checkout: fix performance regression on NFS (r1710167)
    * gpg-agent: properly handle passwords with percent characters (issue #4611)
    * svn-graph.pl: fix assertion about a non-canonical path (r1729060 et al)
    * hot-backup.py: better input validation (r1721174, r1721175)
    * commit: abort on Ctrl-C in plaintext password prompt (issue #4624)
    * diff: produce proper forward binary diffs with --git (r1704292, r1704573)
    * ra_serf: fix deleting directories with many files (issue #4557)

  - Server-side bugfixes:
    * improve documentation for AuthzSVNGroupsFile and groups-db (r1730856)
    * fsfs: reduce peak memory usage when listing large directories (r1725180)
    * fsfs: fix a rare source of incomplete dump files and reports (r1717876)

  - Client-side and server-side bugfixes:
    * update INSTALL documentation file (r1703470 et al)
    * fix potential memory access bugs (r1722860 et al)
    * fix potential out of bounds read in svn_repos_get_logs5() (r1738259)

  - Bindings bugfixes:
    * ignore absent nodes in javahl version of svn status -u (r1720643)

Developer-visible changes:
  - General:
    * fix ruby test suite to work with test-unit gem (r1714790)
    * allow building against KDE4 without conflict with KDE5 (r1734926)
    * fix update_tests.py#76 with SVNPathAuthz short_circuit (r1736432)
    * build system tweaks:
    * tweak how symbolic error names in maintainer mode (r1735179)
    * fix inconsistent behavior of inherited property API (r1717874 et al)

  - API changes:
    * properly interpret parameters in svn_wc_get_diff_editor6() (r1728308)

(ryoon)