sync-all should have a resume option

[ezyang]

If sync-all is run with flags that can cause it to fail given some problem (--ff-only, --rebase, etc.) it would be convenient if the command fails on some repo for it to allow us to resume from that repo (so we don't have to wait for all the repos that we know are fine.) Something like writing out the last repo to a file which is read out when --resume is called.

[igloo@…]

commit bcda801d61c48f536af53b75815d4d2bd2abded4
Author: Ian Lynagh <igloo@earth.li>
Date:   Fri Jul 15 22:14:33 2011 +0100

    Add --resume support to sync-all; trac #5119
    
    Now after, for example, "./sync-all pull", if pulling in one repo fails
    then you can restart from that point with "./sync-all --resume pull".

[igloo]

Fixed.