completion: perform DWIM logic directly in __git_complete_refs

completion: perform DWIM logic directly in __git_complete_refs

__git_complete_refs is the main function used for completing references.
It is primarily used as a wrapper around __git_refs, and is easier to
extend since its arguments are option-like.

One major downside of __git_complete_refs and __git_refs currently, is
the lack of ability to complete only a subset of refs such as branches
(refs/heads) or tags (refs/tags).

Normally, a caller might just decide to use __git_heads() or
__git_tags(). However, in the case of git-switch, it is useful to
complete both branches *and* DWIM remote branch names.

Due to the complexity and implementation of __git_refs, it is not easy
to extend it to support listing only a subset of references.

Instead, we can extend __git_complete_refs to do this. For this to be
done, we must first ensure that "--dwim" support is not tied to calling
__git_refs.

Instead of passing $dwim into __git_refs, we can implement
a __gitcomp_direct_append function which can append to COMPREPLY after
a call to __gitcomp_direct.

If --dwim is passed to __git_complete_refs, use __gitcomp_direct_append
to add the output of __git_dwim_remote_heads to the completion list.

In this way, --dwim support is now independent of calling __git_refs.

A future change will add an additional option to control what set of
references __git_complete_refs will output.

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