git-svn: fix multi-init

git-svn: fix multi-init

After the bugfix to connect to repositories where the user has
limited read permissions, multi-init was broken due to our
SVN::Ra connection being limited to working in a subdirectory;
so we now create a new Ra connection for init-ing branches
and another for tags

Along with that fix, allow the user to use the command-line
option flags for multi-init (--revision being the most notable;
but also --no-auth-cache, --config-dir, --username (for passing
to SVN), and --shared/--template for passing to git-init-db

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