Fix shellcheck warnings

And disable them when necessary. This is all dumb and terrible and I
hate this world and I really really should've used a real programming
language instead of shell why did I do this to myself.

update-ref: index is a normal ref now

resolve-ref: initial commit

Replaces resolve_ref in lib.sh

update-tree: create nonexistent files/directories

update-tree: initial commit

Fix handling of ambiguous ref prefixes

insert: handle trees

Rewrite in POSIX sh

Also don't compress objects and make a few other small changes to the
spec.