git-svn: fix termination issues for remote svn connections

git-svn: fix termination issues for remote svn connections

git-svn used in combination with serf to talk to svn repository
served over HTTPS dumps core on termination.

This is caused by a bug in serf, and the most recent serf release
1.3.1 still exhibits the problem; a fix for the bug exists (see
https://code.google.com/p/serf/source/detail?r=2146).

Until the bug is fixed, work around the issue within the git perl
module Ra.pm by freeing the private copy of the remote access object
on termination, which seems to be sufficient to prevent the error
from happening.

Note: Since subversion-1.8.0 and later do require serf-1.2.1 or
later, this issue typically shows up when upgrading to a recent
version of subversion.

Credits go to Jonathan Lambrechts for proposing a fix to Ra.pm,
Evgeny Kotkov and Ivan Zhakov for fixing the issue in serf and
pointing me to that fix.

Signed-off-by: Uli Heller <uli.heller@daemons-point.com>
Tested-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>