completion: fix and update 'git log --decorate=' options

'git log --decorate=' understands the 'full', 'short' and 'no' options.
From these the completion script only offered 'short' and it offered
'long' instead of 'full'.
Signed-off-by: NSZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1181,7 +1181,7 @@ _git_log ()
 		return
 		;;
 	--decorate=*)
-		__gitcomp "long short" "" "${cur##--decorate=}"
+		__gitcomp "full short no" "" "${cur##--decorate=}"
 		return
 		;;
 	--*)