bash completion: Add '--merge' long option for 'git log'

Signed-off-by: NLee Marlow <lee.marlow@gmail.com>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -972,6 +972,7 @@ _git_log ()
 			--decorate --diff-filter=
 			--color-words --walk-reflogs
 			--parents --children --full-history
+			--merge
 			"
 		return
 		;;