clone: handle unborn branch in bare repos

clone: handle unborn branch in bare repos

When cloning a repository with an unborn HEAD, we'll set the local HEAD
to match it only if the local repository is non-bare. This is
inconsistent with all other combinations:

  remote HEAD       | local repo | local HEAD
  -----------------------------------------------
  points to commit  | non-bare   | same as remote
  points to commit  | bare       | same as remote
  unborn            | non-bare   | same as remote
  unborn            | bare       | local default

So I don't think this is some clever or subtle behavior, but just a bug
in 4f37d45706 (clone: respect remote unborn HEAD, 2021-02-05). And it's
easy to see how we ended up there. Before that commit, the code to set
up the HEAD for an empty repo was guarded by "if (!option_bare)". That's
because the only thing it did was call install_branch_config(), and we
don't want to do so for a bare repository (unborn HEAD or not).

That commit put the handling of unborn HEADs into the same block, since
those also need to call install_branch_config(). But the unborn case has
an additional side effect of calling create_symref(), and we want that
to happen whether we are bare or not.

This patch just pulls all of the "figure out the default branch" code
out of the "!option_bare" block. Only the actual config installation is
kept there.

Note that this does mean we might allocate "ref" and not use it (if the
remote is empty but did not advertise an unborn HEAD). But that's not
really a big deal since this isn't a hot code path, and it keeps the
code simple. The alternative would be handling unborn_head_target
separately, but that gets confusing since its memory ownership is
tangled up with the "ref" variable.

There's just one new test, for the case we're fixing. The other ones in
the table are handled elsewhere (the unborn non-bare case just above,
and the actually-born cases in t5601, t5606, and t5609, as they do not
require v2's "unborn" protocol extension).

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