I don't know the details about sa_serf. I guess it is a new DAV
library for subversion client, to replace ra_neon have used for a long
time.
Here is a brief description about sa_serf, from
http://subversion.apache.org/docs/release-notes/1.5.html#dav-modules
"Subversion 1.4 introduced the experimental ra_serf repository
access module for accessing HTTP[S] DAV Subversion servers. This uses
the serf library instead of the Neon library which the original DAV
support uses. serf supports pipelined requests which may lead to
better performance. However, Subversion 1.4 required you to choose
which module to use for accessing DAV servers at build time, which
made it difficult to find out which module performs better for your
usage patterns.
Subversion 1.5 allows you to build both modules at the same time;
you can choose which library to use on a global or host-by-host basis
by setting the http-library variable in your run-time server
configuration file (~/.subversion/servers). In recognition of the fact
that both libraries are DAV clients, we have renamed ra_dav to
ra_neon."