clone: move unborn head creation to update_head()

clone: move unborn head creation to update_head()

Prior to 4f37d45706 (clone: respect remote unborn HEAD, 2021-02-05),
creation of the local HEAD was always done in update_head(). That commit
added code to handle an unborn head in an empty repository, and just did
all symref creation and config setup there.

This makes the code flow a little bit confusing, especially as new
corner cases have been covered (like the previous commit to match our
default branch name to a non-HEAD remote branch).

Let's move the creation of the unborn symref into update_head(). This
matches the other HEAD-creation cases, and now the logic is consistently
separated: the main cmd_clone() function only examines the situation and
sets variables based on what it finds, and update_head() actually
performs the update.

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