Renamed script flag -Z/--depth -> -z/--depth

Previously, the intention of upper case -Z was the match -W/--width and
-H/--height, which are uppercase to avoid conflicts with -h/--help.

But -z/--depth isn't _really_ related to -W/-H.

This avoids a conflict with -Z/--lebesgue, but may conflict with
-z/--cat. Fortunately we don't currently have any conflicts with the
latter. Since -z/--depth and -Z/--lebesgue are both disk-layout related,
the risk of conflicts are probably much higher there.

scripts/perfbd.py

@@ -1235,7 +1235,7 @@ if __name__ == "__main__":
         help="Depth to propagate samples up the call-stack. 0 propagates up "
             "to the entry point, 1 does no propagation. Defaults to 0.")
     parser.add_argument(
-        '-Z', '--depth',
+        '-z', '--depth',
         nargs='?',
         type=lambda x: int(x, 0),
         const=0,