completion: don't set PARSE_OPT_NOCOMPLETE on --rerere-autoupdate

There is not a strong reason to hide this option, and git-merge already
completes this one. Let's allow to complete this for all commands (and
let git-completion.bash do the suppressing if needed).

This makes --rerere-autoupdate completable for am, cherry-pick and
revert. rebase completion is fixed manually because it's a shell
script and does not benefit from --git-completion-helper.

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

contrib/completion/git-completion.bash

@@ -1754,8 +1754,7 @@ _git_merge ()
 
 	case "$cur" in
 	--*)
-		__gitcomp_builtin merge "--rerere-autoupdate
-				--no-rerere-autoupdate
+		__gitcomp_builtin merge "--no-rerere-autoupdate
 				--no-commit --no-edit --no-ff
 				--no-log --no-progress
 				--no-squash --no-stat
@@ -1963,6 +1962,7 @@ _git_rebase ()
 			--autostash --no-autostash
 			--verify --no-verify
 			--keep-empty --root --force-rebase --no-ff
+			--rerere-autoupdate
 			--exec
 			"