taskd/clone.sh: exponential backoff for git-svn retries

taskd/clone.sh: exponential backoff for git-svn retries

When `git-svn fetch` fails and makes no progress, double the
delay between attempts as long there is no progress made.

If any progress has been made, reset the delay back to
the minimum.

Increase the number of consecutive no-progress retries from
five to seven since the wait will be much longer between retries.

Since no retry can take place unless the server has been
successfully contacted and the initial branch layout set up,
any invalid `git-svn` clone source URL will still fail
immediately regardless of this change.

Additionally, since the update process (i.e. jobd/update.sh)
does *NOT* perform any delay+retry (rather it's left up to
the normal jobd.pl update retry mechanism), there's no impact
to jobd.pl operation with this change.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>