taskd/clone.sh: persistently attempt git-svn clones
During the initial clone, git-svn has a nasty habit of dropping
out with an error before the clone has completed, especially if
the source repository is large.
However, git-svn fetch will pick up where it left off if it's
just run again.
As long as we continue to make progress, keep restarting the
fetch until the clone has fully completed.
Initial svn clones of large repositories will now succeed much
more reliably than before.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>