git-svn: introduce SVN version comparison function

git-svn: introduce SVN version comparison function

With double-digit version components in SVN::Core::VERSION,
a naive string comparison is no longer sufficient and
a numeric comparison is required for correct results.

This fixes a regression introduced in
commit 082afee621aeb2d3746c8ae290af98823f981f34
("git-svn: use platform specific auth providers")
where SVN version "1.6.6" was incorrectly assumed to
be newer than the required "1.6.12" version.

[mk: fix namespace references]
[ew: commit message]

Tested-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>