completion: handle other variants of "branch -m"

completion: handle other variants of "branch -m"

We didn't special-case "branch -M" (with a capital M) the same as
"branch -m", nor any of the "--copy" variants. As a result these offered
any ref as the next candidate, and not just branch names.

Note that I rewrapped case-arm line since it's now quite long, and
likewise the one below it for consistency. I also re-ordered the
existing "-D" to make it more obvious how the cases group together.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>