builtin/worktree: move setup of commondir file earlier

builtin/worktree: move setup of commondir file earlier

Shuffle around how we create supporting worktree files so that we first
ensure that the worktree has all link files ("gitdir", "commondir")
before we try to initialize the ref database by writing "HEAD". This
will be required by a subsequent commit where we start to initialize the
ref database via `refs_init_db()`, which will require an initialized
`struct worktree *`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>