completion(switch/checkout): treat --track and -t the same

completion(switch/checkout): treat --track and -t the same

When `git switch --track ` is to be completed, only remote refs are
eligible because that is what the `--track` option targets.

And when the short-hand `-t` is used instead, the same _should_ happen.
Let's make it so.

Note that the bug exists both in the completions of `switch` and
`completion`, even if it manifests in slightly different ways: While
the completion of `git switch -t ` will not even look at remote refs,
the completion of `git checkout -t ` will look at both remote _and_
local refs. Both should look only at remote refs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>