setup: make repository discovery self-contained

setup: make repository discovery self-contained

In the preceding commits we have introduced a separate repository
discovery phase and refactored the logic so that we have two clear
phases:

  1. Repository discovery, which doesn't modify the repository itself at
     all.

  2. Repository configuration, which takes the information we have
     discovered to set up the repository.

Extract the first phase into a new function `repo_discover()` to further
stress these two different phases.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>