completion: complete missing 'git log' options

Some options specific to 'git log' are missing from the Bash completion
script. Add them to _git_log.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2180,6 +2180,8 @@ _git_log ()
 			--no-walk --no-walk= --do-walk
 			--parents --children
 			--expand-tabs --expand-tabs= --no-expand-tabs
+			--clear-decorations --decorate-refs=
+			--decorate-refs-exclude=
 			$merge
 			$__git_diff_common_options
 			"