bash: add the merge option --ff-only

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1224,7 +1224,7 @@ _git_log ()
 
 __git_merge_options="
 	--no-commit --no-stat --log --no-log --squash --strategy
-	--commit --stat --no-squash --ff --no-ff
+	--commit --stat --no-squash --ff --no-ff --ff-only
 "
 
 _git_merge ()