setup: unify repository version callbacks

setup: unify repository version callbacks

Once upon a time, check_repository_format_gently would parse
the config with a single callback, and that callback would
set up a bunch of global variables. But now that we have
separate workdirs, we have to be more careful. Commit
31e26eb (setup.c: support multi-checkout repo setup,
2014-11-30) introduced a reduced callback which omits some
values like core.worktree. In the "main" callback we call
the reduced one, and then add back in the missing variables.

Now that we have split the config-parsing from the munging
of the global variables, we can do it all with a single
callback, and keep all of the "are we in a separate workdir"
logic together.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>