x/build: can't test talks repo anymore

[bradfitz]

Gerrit is replying with:

2017-02-17T19:31:48Z finish_get_source_from_gerrit after 2.392597208s; err=reading talks/9e62c07834496e644aab335b13664537be358557 from gerrit: body over 26214400 bytes; talks-9e62c07834496e644aab335b13664537be358557 from gerrit

.... when trying to test the "talks" repo.

  2017-02-17T19:31:37Z finish_get_buildlet after 54.443317519s
  2017-02-17T19:31:37Z using_buildlet 10.240.0.21:80
  2017-02-17T19:31:37Z write_snapshot_tar 
  2017-02-17T19:31:46Z finish_write_snapshot_tar after 9.24692393s
  2017-02-17T19:31:46Z make_and_test 
  2017-02-17T19:31:46Z fetching_subrepo talks
  2017-02-17T19:31:46Z get_source 
  2017-02-17T19:31:46Z get_source_from_gerrit talks-9e62c07834496e644aab335b13664537be358557 from gerrit
  2017-02-17T19:31:48Z finish_get_source_from_gerrit after 2.392597208s; err=reading talks/9e62c07834496e644aab335b13664537be358557 from gerrit: body over 26214400 bytes; talks-9e62c07834496e644aab335b13664537be358557 from gerrit
  2017-02-17T19:31:48Z finish_get_source after 2.45508646s; err=reading talks/9e62c07834496e644aab335b13664537be358557 from gerrit: body over 26214400 bytes
  2017-02-17T19:31:48Z finish_make_and_test after 2.514948252s; err=runTests: reading talks/9e62c07834496e644aab335b13664537be358557 from gerrit: body over 26214400 bytes

Not sure why the coordinator is not trying to use the gitmirror, though.

/cc @campoy