spec: assert(io.open()) to fail early

hglib: have an error message for unexpected data on required channel

hglib: consume command as one argument to runcommand() (allows other arguments)

.luacheckrc: use std=min (in hope of supporting Lua 5.2+ some day)

pipelines: missing dependency for log example

pipelines: also test the new log example

examples: no need to update working directory when using just log

hglib: write proper Lua! Yess!

examples: add log -Tjson example

spec: client should also close file handles on :close()