clar: add tap output option

diff::parse: don't include `diff.h`

We don't call any internal functions in the test; we don't need to
include `../src/diff.h`.

release script: fix typo

Add an output abstraction layer, with a single output format, "clap",
the clar protocol, which is the current output format for clar.

We want to parse arguments before we start printing any output; the
arguments themselves may impact the way we display that output.

tests: offer exact name matching with a `$` suffix