completion: add tests showing subpar switch/checkout --track logic

completion: add tests showing subpar switch/checkout --track logic

When the --track option is provided to git switch or git checkout, and
no branch is specified by -c or -b, git will interpret the tracking
branch to determine the local branch name to use. This "Do What I Mean"
logic is similar but distinct from the default DWIM logic of
interpreting a unique remote branch name as a request to create and
track that branch.

For example, `git switch --track origin/master` is interpreted as
a request to create a local branch named master that is tracking
origin/master.

The current completion for git checkout in this regard is only somewhat
poor:

 $git checkout --track <TAB>
 HEAD
 master
 matching-branch
 matching-tag
 other/branch-in-other
 other/master-in-other

At least it still includes remote references. The clutter from including
all references isn't too bad.

However, git switch completion is terrible:

 $git switch --track <TAB>
 master
 matching-branch

It only shows local branches, not even allowing any form of completion
of the remote references!

Add tests which highlight the expected behavior of completing --track on
its own.

Note that when -c/-C or -b/-B are provided we do expect completing more
references, but this will be discussed in a future change that addresses
these options specifically.

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