Add shellcheck to gitlab ci and git hook

This is a resubmission of !270 (merged) (apropos #304 (closed)), a contribution by @schischkin.

I'm making a new MR of it because something seems to have happened to https://gitlab.torproject.org/schischkin/arti which prevented me from adding a tiny fixup to the branch. Otherwise I would have merged it.

changed the description

enabled an automatic merge when the pipeline for 7f83b9fb succeeds

mentioned in merge request !270 (merged)

merged

mentioned in commit 880d5f2d

mentioned in issue #304 (closed)