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.

init: use commit instead of commit-tree

init: add .tmignore in initial commit

init: set $TM_DIR before sourcing lib.sh

init: minor fixups

init: handle init in existing repo

Also create an initial HEAD commit.

Implement .tmignore

Make index a normal ref

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.