git-svn: use ~/.subversion config files when using SVN:: libraries

git-svn: use ~/.subversion config files when using SVN:: libraries

This allows users to use HTTP proxy information (among other settings)
from ~/.subversion/servers and ~/.subversion/config

--config-dir (as before) may be passed to git-svn to override the
default choice of '~/.subversion' for the configuration directory.

Thanks to tko on #git for pointing this out.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>