completion: complete values for help.format

Signed-off-by: NStephen Boyd <bebarino@gmail.com>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1348,6 +1348,10 @@ _git_config ()
 			"
 		return
 		;;
+	help.format)
+		__gitcomp "man info web html"
+		return
+		;;
 	*.*)
 		COMPREPLY=()
 		return