Do incremental SVN checkouts

In hydra-evaluator, reuse an SVN working copy between runs (similar to
what we do with Git and other input types).  This reduces network
traffic in the common case.

Also, don't use nix-prefetch-svn.  It doesn't do anything useful.

src/script/Makefile.am

@@ -1,9 +1,5 @@
 EXTRA_DIST = \
   hydra-control \
-  nix-prefetch-svn \
-  nix-prefetch-git \
-  nix-prefetch-bzr \
-  nix-prefetch-hg \
   $(bin_SCRIPTS)
 
 bin_SCRIPTS = \
@@ -13,7 +9,6 @@ bin_SCRIPTS = \
   hydra-server \
   hydra-update-gc-roots \
   hydra-create \
-  nix-prefetch-svn \
   nix-prefetch-git \
   nix-prefetch-bzr \
   nix-prefetch-hg