sort-revset: introduce a `random` variant This new `sort` variant allows to shuffle any revset. It also allow for randomly picking element using `first`.

tests: remove flakiness in test-nointerrupt.t The problem was that the reaction to the signal was racing against the completion of the command. Since reaction to the signal is to print a line of warning, we can fix this by waiting for that warning to appear before allowing the command to complete.