This returns a diff-able human-readable version of obj. This
differs from the regular mu_pp because it expands escaped
newlines and makes hex-values generic (like object_ids). This uses mu_pp to do the first pass and then cleans it up.
# File lib/minitest/unit.rb, line 165
def mu_pp_for_diff obj # TODO: possibly rename
mu_pp(obj).gsub(/\\n/, "\n").gsub(/0x[a-f0-9]+/, '0xXXXXXX')
end