submodule: print graph output next to submodule log

When running "git log -p --submodule=log", the submodule log is not
indented by the graph output, although all other lines are.  Fix this by
prepending the current line prefix to each line of the submodule log.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2255,6 +2255,7 @@ static void builtin_diff(const char *name_a,
 		const char *del = diff_get_color_opt(o, DIFF_FILE_OLD);
 		const char *add = diff_get_color_opt(o, DIFF_FILE_NEW);
 		show_submodule_summary(o->file, one ? one->path : two->path,
+				line_prefix,
 				one->sha1, two->sha1, two->dirty_submodule,
 				meta, del, add, reset);
 		return;