pretty: pass graph width to pretty formatting for use in '%>|(N)'

Pass graph width to pretty formatting, to make N in '%>|(N)'
include columns consumed by graph rendered when --graph option
is in use.

For example, in the output of

  git log --all --graph --pretty='format: [%>|(20)%h] %ar%d'

this change will make all commit hashes align at 20th column from
the edge of the terminal, not from the edge of the graph.

Signed-off-by: Josef Kufner <josef@kufner.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -161,6 +161,7 @@ struct pretty_print_context {
 	 * should not be counted on by callers.
 	 */
 	struct string_list in_body_headers;
+	int graph_width;
 };
 
 struct userformat_want {