Completion: Add support for --submodule=diff

Teach git-completion.bash about the 'diff' option to 'git diff
--submodule=', which was added in Git 2.11.
Signed-off-by: NPeter Law <PeterJCLaw@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1206,7 +1206,7 @@ _git_describe ()
 
 __git_diff_algorithms="myers minimal patience histogram"
 
-__git_diff_submodule_formats="log short"
+__git_diff_submodule_formats="diff log short"
 
 __git_diff_common_options="--stat --numstat --shortstat --summary
 			--patch-with-stat --name-only --name-status --color