completion: improve handling of --detach in checkout

completion: improve handling of --detach in checkout

Just like git switch, we should not complete DWIM remote branch names
if --detach has been specified. To avoid this, refactor _git_checkout in
a similar way to _git_switch.

Note that we don't simply clear dwim_opt when we find -d or --detach, as
we will be adding other modes and checks, making this flow easier to
follow.

Update the previously failing tests to show that the breakage has been
resolved.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>