HTTP repository sync sometimes fails due to authentication issues

Seen on gprd: gitlab-org/gitlab-ce is 8 hours behind, and attempts to synchronize are failing with the message fatal: could not read Username for 'https://gitlab.com': terminal prompts disabled

@stanhu points at https://109d3390fac34f3248388dc3d64a49da.us-central1.gcp.cloud.es.io:9243/goto/0053f1d21e771489fdd0f7e6472b1637

This suggests that the JWT authentication we're writing to the configuration file is expiring before the git fetch triggers (bug), or there is a misconfiguration like mismatched clocks. Needs further investigation.

Other syncs are proceeding apace, so it seems unlikely to be a general problem.

/cc @stanhu