bash completion: Add more long options for 'git log'

Options added: --parents --children --full-history
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

@@ -848,6 +848,7 @@ _git_log ()
 			--stat --numstat --shortstat
 			--decorate --diff-filter=
 			--color-words --walk-reflogs
+			--parents --children --full-history
 			"
 		return
 		;;