completion: improve handling of --track in switch/checkout

completion: improve handling of --track in switch/checkout

Current completion for the --track option of git switch and git checkout
is sub par. In addition to the DWIM logic of a bare branch name, --track
has DWIM logic to convert specified remote/branch names into a local
branch tracking that remote. For example

  $git switch --track origin/master

This will create a local branch name master, that tracks the master
branch of the origin remote.

In fact, git switch --track on its own will not accept other forms of
references. These must instead be specified manually via the -c/-C/-b/-B
options.

Introduce __git_remote_heads() and the "remote-heads" mode for
__git_complete_refs. Use this when the --track option is provided while
completing in _git_switch and _git_checkout. Just as in the --detach
case, we never enable DWIM mode for --track, because it doesn't make
sense.

It should be noted that completion support is still a bit sub par when
it comes to handling -c/-C and --orphan. This will be resolved in
a future change.

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