Handle a nonexistent repository with a 404 error.
Based on changeset 62ea868cafbfa3acb7e5dba6982ecb46a1637246 from
Vincent Wagelaar <vincent@ricardis.tudelft.nl>
in http://hannibal.lr-s.tudelft.nl/~vincent/fcgi
removing bthis update spans a branch affecting the following files: baborting update spanning branches!(use update -m to merge across branches or -C to lose changes)