worktree: avoid resolving HEAD unnecessarily

worktree: avoid resolving HEAD unnecessarily

Now that git-worktree sets HEAD explicitly to its final value via either
git-symbolic-ref or git-update-ref, rather than relying upon
git-checkout to do so, the "hack" for pacifying is_git_directory() with
a temporary HEAD, though still necessary, can be simplified.

Since the real HEAD is now populated with its proper final value, the
value of the temporary HEAD truly no longer matters, and any value which
looks like an object ID is good enough to satisfy is_git_directory().
Therefore, just set the temporary HEAD to a literal value rather than
going through the effort of resolving the current branch's HEAD.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>