A burst of git calls no longer holds up the server

Obeya runs many git calls at once: one in each clone, plus landings and fetches. Starting a process used to block the server's own thread, and on a busy Windows machine each start could take seconds, so Obeya stopped answering pages and phones for 10–20 seconds at a time. Processes now start on two helper threads, and the server keeps answering while they start. The page shows measurements from a busy Windows machine and from a Mac.