git-svn: ignore expansion of svn:keywords

Unlike my earlier test patch, this also checks svn:eol-style and
makes sure it's applied to working copy updates.  This is
definitely more correct than my original attempt at killing
keyword expansions, but I still haven't tested it enough to
know.  Feedback would be much appreciated.

Also changed assert_svn_wc_clean() to only work on the svn
working copy.  This requires a separate call to assert_tree() to
check wc integrity against git in preparation for another change
I'm planning.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>