This way we can avoid the spawning of a new SVN::Ra session by
reusing the existing one.

The most problematic issue is that some svn servers disallow
too many connections from a single IP, so this will allow
git-svn to fetch from those repositories with a higher success
rate by using fewer connections.

This sometimes showed up as a new (and redundant)
[svn-remote "$parent_refname"] entry in $GIT_DIR/svn/.metadata.
Signed-off-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>