git-svn: remove support for the svn command-line client

Using the command-line client was great for prototyping and
getting something working quickly.  Eventually I found time
to study the library documentation and add support for using
the libraries which are much faster and more flexible when
it comes to supporting new features.

Note that we require version 1.1 of the SVN libraries, whereas
we supported the command-line svn client down to version 1.0.

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