t/helper: stop setting up `the_repository` repeatedly

t/helper: stop setting up `the_repository` repeatedly

The "repository" test helper sets up `the_repository` twice. In fact
though, we don't even have to set it up even once: all we need is to set
up its hash algorithm, because we still depend on some subsystems that
aren't free of `the_repository`.

Refactor the code accordingly. This prepares for a subsequent change,
where setting up the repository repeatedly will lead to a `BUG()`.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>