setup: document check_repository_format()

setup: document check_repository_format()

This function's interface is rather enigmatic, so let's
document it further.

While we're here, let's also drop the return value. It will
always either be "0" or the function will die (consequently,
neither of its two callers bothered to check the return).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>