contrib/git-svn: fix a copied-tree bug in an overzealous assertion

contrib/git-svn: fix a copied-tree bug in an overzealous assertion

I thought passing --stop-on-copy to svn would save us from all
the trouble svn-arch-mirror had with directory (project) copies.
I was wrong, there was one thing I overlooked.

If a tree was moved from /foo/trunk to /bar/foo/trunk with no
other changes in r10, but the last change was done in r5, the
Last Changed Rev (from svn info) in /bar/foo/trunk will still be
r5, even though the copy in the repository didn't exist until
r10.

Now, if we ever detect that the Last Changed Rev isn't what
we're expecting, we'll run svn diff and only croak if there are
differences between them.

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