git-svn testsuite: use standard configuration for Subversion tools

git-svn testsuite: use standard configuration for Subversion tools

I have tweaked configuration in my ~/.subversion directory, namely I am
running auto-properties and automatically adding '$Id$' expansion to
every file.  This choke the last test named 'proplist' from
t9101-git-svn-props.sh, because one more property, svn:keywords is
automatically added.

I had just wrapped svn invocation with the svn_cmd that specifies empty
directory via --config-dir argument.  Since the latter is the global
option, it should be recognized by all svn subcommands, so no
regressions will be introduced.

Now svn_cmd is used everywhere, not just in the failed test module: this
should guard us from the future clashes with user-defined configuration
tweaks.

Signed-off-by: Eygene Ryabinkin <rea-git@codelabs.ru>
Acked-by: Eric Wong <normalperson@yhbt.net>