git-svn: multi-init saves and reuses --tags and --branches arguments

This should make it much easier to track newly added tags and
branches.  Re-running multi-init without command-line arguments
should now detect new-tags and branches.

--trunk shouldn't change often, but running multi-init on it
is now idempotent.

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