diffstat: print 0 instead of nothing for 0 adds or removes

This is in line with how the stock diffstat behaves when processing a
single diff with no line modifications (like a binary diff).

mercurial/patch.py

@@ -1407,7 +1407,7 b' def diffstat(lines, width=80):'
     for filename, adds, removes in stats:
         pluses = '+' * scale(adds)
         minuses = '-' * scale(removes)
-        output.append(' %-*s |  %*.d %s%s\n' % (maxname, filename, countwidth,
+        output.append(' %-*s |  %*s %s%s\n' % (maxname, filename, countwidth,
                                                 adds+removes, pluses, minuses))
 
     if stats: