setup: propagate prefix via repository discovery

setup: propagate prefix via repository discovery

In the preceding commits we have started to propagate all information
required for the configuration of the repository via a new `struct
repo_discovery`. The only exception is the repository's prefix, which we
still return via the return parameter.

This is conceptually fine, but somewhat inconsistent. Refactor this to
instead propagate the prefix via the repository discovery, too.

While at it, drop a static variable in `repo_discover_bare_gitdir()`.
We apply its value to the repository discovery anyway, so we don't have
to keep it around afterwards anymore.

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