SVN plugin: Never launch configured external diff and diff3 tools.

I have my command-line Subversion client set up to use colordiff
instead of the internal diff implementation, so of course instead of
seeing a nice diff in KDevelop, I got a mess of ANSI colour escape
codes.

This patch disables any external diff or diff3 tools configured in
the ~/.subversion/config file. It does this by modifying
configuration in memory right after it is parsed from disk (or
possibly the registry on Windows). This way, we can safely assume
that running a diff or merge will produce unidiff format, and the
config file doesn't have to be changed.

N.B., it may be necessary to override the merge-tool command
(SVN_CONFIG_OPTION_MERGE_TOOL_CMD) in the same way;

REVIEW: 115444