setup: introduce explicit repository discovery

setup: introduce explicit repository discovery

When setting up the global repository we intermix repository discovery
and repository configuration: we repeatedly call `set_git_work_tree()`
and `apply_and_export_relative_gitdir()` until we're happy with the
result. The result of this is then a partially-configured repository
that we use for further setup.

This process is quite hard to follow, as it's never quite clear which
parts of the repository have been configured already and which haven't.
Furthermore, it means that the repository configuration is distributed
across many different places instead of having it neatly contained in a
single location. Ultimately, this is the reason that we cannot use a
central function like `repo_init()`.

Refactor the logic so that we stop partially-configuring a repository
and instead populate a new `struct repo_discovery`. This allow us to
essentially split repository setup into two phases:

  - The first phase only figures out parameters required to configure
    the repository.

  - The second phase then takes these parameters and applies them to the
    repository.

Like this, we'll never end up with a partially-configured repository and
can eventually extend `repo_init()` to handle the full initialization
for us.

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