Fixed long-standing and very subtle bug in ultratb.
Fixed long-standing and very subtle bug in ultratb.

This is something I had seen sporadically, and was able to finally pin
down as associated with situations like generator expressions and
others where inspect.getargvalues() returns variables that
inspect.formatargvalues() later can't extract.  I finally understood
what's going on, left a detailed comment in the code explaining the
problem.

ipython/ipython,ipython/ipython