graphlog: attempt to fix index overrun (issue2912)

This bug may be caused by file subgraphs have more than two parents
per node. I have no idea if this fix is correct as the graphlog code
is mysterious, but it seems to be fine on the available test case.

hgext/graphlog.py

@@ -95,6 +95,8 b' def draw_edges(edges, nodeline, interlin'
         elif start == end:
             interline[2 * start] = "|"
         else:
+            if 2 * end >= len(nodeline):
+                continue
             nodeline[2 * end] = "+"
             if start > end:
                 (start, end) = (end, start)