commands: actually implement --closed for topological heads

commands: externalize branchheads so we can do it for all branches at once

commands: move inactive/closed out so they can have more effect

commands: don't do too much work for error messages

commands: allow --closed even for topological heads displays But it actually only starts working later on.

util: make spawndetached() handle subprocess early terminations The file-based synchronization introduced by e22695b4472f hangs when the child process fails before terminating the handshake, which the previous pipe-based version handled correctly. To fix this, the parent polling loop was fixed to detect premature terminations of the child process.