RB stablization: experiment: reduce diffoscope recursion limit

Reduces the diffoscope container depth limit (how many levels of a file that is a container for another file, and so on) will be unpacked and compared -- from the default of 50 down to 3.

This is an experiment to see whether this helps reduce the number of OOM errors on the Jenkins host. The working theory that it tests is that there may be a small number of pathological comparison cases that recurse deeply and consume large amounts of memory.

From experience debugging individual package differences by reading the HTML output of diffoscope, it's relatively rare for files to contain more than three or four levels of nested containers.

This option became available in diffoscope v100 in September of Y2018 so should be safe for usage here (current required version of diffoscope is v202 in the RB script).

Relates-to #1059483 (but doesn't fix it).