[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 <redacted>
Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 v2 simplifies the code. $nocache in v1 was overkill.

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

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 9f71bcde96..8c6b610a24 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash

@@ -400,7 +400,8 @@ __gitcomp_builtin ()
 	if [ -z "$options" ]; then
 		# leading and trailing spaces are significant to make
 		# option removal work correctly.
-		options=" $incl $(__git ${cmd/_/ } --git-completion-helper) "
+		options=" $incl $(__git ${cmd/_/ } --git-completion-helper) " || return
+
 		for i in $excl; do
 			options="${options/ $i / }"
 		done

-- 
2.22.0.rc0.322.g2b0371e29a