queue: only git fetch once per minute per branch
queue: only git fetch once per minute per branch

This takes 1-2 seconds and makes launching jobs slow.  Only do it once
every 60 seconds per branch.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@inktank.com>

dreamhost/teuthology,dreamhost/teuthology,t-miyamae/teuthology,robbat2/teuthology,t-miyamae/teuthology,caibo2014/teuthology,ktdreyer/teuthology,robbat2/teuthology,ceph/teuthology,SUSE/teuthology,SUSE/teuthology,zhouyuan/teuthology,michaelsevilla/teuthology,caibo2014/teuthology,ivotron/teuthology,dmick/teuthology,michaelsevilla/teuthology,tchaikov/teuthology,yghannam/teuthology,SUSE/teuthology,ceph/teuthology,zhouyuan/teuthology,dmick/teuthology,ivotron/teuthology,yghannam/teuthology,ktdreyer/teuthology,tchaikov/teuthology,dmick/teuthology