Merge branch 'nd/completion-no-cache-failure' into maint

An incorrect list of options was cached after command line
completion failed (e.g. trying to complete a command that requires
a repository outside one), which has been corrected.

* nd/completion-no-cache-failure:
  completion: do not cache if --git-completion-helper fails

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