git-svn: allow 'init' to act as multi-init

git-svn: allow 'init' to act as multi-init

multi-init is now just an alias that requires -T/-t/-b;
all options that 'init' can now accept.

This will hopefully simplify usage and reduce typing.

Also, allow the --shared option in 'init' to take an optional
argument now that 'git-init --shared' supports an optional
argument.

Signed-off-by: Eric Wong <normalperson@yhbt.net>