sync: Now supports remote groups, the same way git remote update does.
fix addurl behavior when location and url logs are inconsistent

The url log could have an url for a key, while the location log thinks it's
not present in the web. In this case, addurl --file url would not do
anything. Fixed it to re-add the web as a location.

I don't know how this situation could arise, but I saw it in the wild in
the conference_proceedings repo, affecting key
URL-s17806003--http://mirror.linux.org.au/pub/linux.conf.au/2014/Wednesday/53-Building_Effective_Alliances_around_the_Trans-Pacific_Partnershi-c0505b631127ccc67e38e637344d988e
Investigating the presence log, it looked like that key
was originally listed as present in the web, then in commit
56abf9e9f3e691ed9d83513037d4019313321ca3 someone else's git-annex
set it and some other things to not present in the web. It would be
interesting to know what that user did, but I doubt I'll be able to find
out. All I can tell from this investigation is that the inconsistency was
not introduced when originally addurl-ing the url.