git-svn: fix several fetch bugs related to repeated invocations

git-svn: fix several fetch bugs related to repeated invocations

We no longer delete the top-level directory even if it got
deleted from the upstream repository.  In gs_do_update; we
double-check that the path we're tracking exists at both
endpoints before proceeding.  We have also added additional
protection against fetching revisions out-of-order.

To simplify our internal interfaces, I've disabled passing the
'recursive' flag to the gs_do_{switch,update} wrapper functions
since we always want it in git-svn.  We also pass the
entire Git::SVN object rather than just the path because it
helped me debug.

When printing progress, the refname is printed out to make
it less confusing when multi-fetch is running.

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