completion: fix multiple command removals

completion: fix multiple command removals

Commit 6532f3740b ("completion: allow to customize the completable
command list", 2018-05-20) tried to allow multiple space-separated
entries in completion.commands. To do this, it copies each parsed token
into a strbuf so that the result is NUL-terminated.

However, for tokens starting with "-", it accidentally passes the
original non-terminated string, meaning that only the final one worked.
Switch to using the strbuf.

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