"git checkout --to" sets up a new working directory with a .git file
pointing to $GIT_DIR/worktrees/<id>. It then executes "git checkout"
again on the new worktree with the same arguments except "--to" is
taken out. The second checkout execution, which is not contaminated
with any info from the current repository, will actually check out and
everything that normal "git checkout" does.
Helped-by: Marc Branchaud <marcnarc@xiplink.com> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>