completion: add dirstat and friends to diff options

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -927,6 +927,8 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--inter-hunk-context=
 			--patience
 			--raw
+			--dirstat --dirstat= --dirstat-by-file
+			--dirstat-by-file= --cumulative
 "
 
 _git_diff ()