completion: do not cache if --git-completion-helper fails

completion: do not cache if --git-completion-helper fails

"git <cmd> --git-completion-helper" could fail if the command checks for
a repo before parse_options(). If the result is cached, later on when
the user moves to a worktree with repo, tab completion will still fail.

Avoid this by detecting errors and not cache the completion output. We
can try again and hopefully succeed next time (e.g. when a repo is
found).

Of course if --git-completion-helper fails permanently because of other
reasons (*), this will slow down completion. But I don't see any better
option to handle that case.

(*) one of those cases is if __gitcomp_builtin is called on a command
  that does not support --git-completion-helper. And we do have a
  generic call

    __git_complete_common "$command"

  but this case is protected with __git_support_parseopt_helper so we're
  good.

Reported-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>