completion: improve completion for git switch with no options

completion: improve completion for git switch with no options

Add a new --mode option to __git_complete_refs, which allows changing
the behavior to call __git_heads instead of __git_refs.

By passing --mode=heads, __git_complete_refs will only output local
branches. This enables using "--mode=heads --dwim" to enable listing
local branches and the remote unique branch names for DWIM.

Refactor completion support to use the new mode option, rather than
calling __git_heads directly. This has the advantage that we can now
correctly allow local branches along with suitable DWIM refs, rather
than only allowing DWIM when we complete all references.

Choose what mode it uses when calling __git_complete_refs. If -d or
--detach have been provided, then simply complete all refs, but
*without* the DWIM option as these DWIM names won't work properly in
--detach mode.

Otherwise, call __git_complete_refs with the default dwim_opt value and
use the new "heads" mode.

In this way, the basic support for completing just "git switch <TAB>"
will result in only local branches and remote unique names for DWIM.

The basic no-options tests for git switch, as well as several of the
-c/-C tests now pass, so remove the known breakage tags.

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