run-tests: "fix" race condition in race condition fix Laurent's commit 3203dfe341f9 still suffers from a race: by the time the "job" function tries to assign to channels[channel], that list has been truncated to empty. The result is that every job thread raises an IndexError. Earlier, I tried an approach of correctly locking channels, but that caused run-tests to hang on KeyboardInterrupt sometimes. This approach is strictly hackier, but seems to actually work reliably.

rebase: restore help for rebase w/o args (issue5059) Restoring documentation accidentally removed in a9a047878e14.