[PATCH v2] 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>
---
 v2 simplifies the code. $nocache in v1 was overkill.

 contrib/completion/git-completion.bash | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)