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.

Fix some errors caught by shellcheck

Replace date +%s with a load of portable garbage

Clear $IFS before read(1)

commit-tree: add type to commit

commit-tree: only show prompt when is a tty

commit, commit-tree: fix options

resolve-ref: initial commit

Replaces resolve_ref in lib.sh

update-tree: create nonexistent files/directories

tm-commit-tree: initial commit