bug traceback should show open objects

[Martin Pool]

It's fairly common that when someone reports a problem with a traceback we also want to know what format and configuration of repository components is involved.

To avoid roundtrips back to the user it would be nice if the traceback included the repr of objects that are involved either by

 * printing all locals like the cgitb module does - this will need some care to stop them getting unreasonably long, and it will anyhow make the traceback considerably longer; maybe too long to print to the screen

 * as a special case keeping a weakref dictionary of open repositories and other interesting things

[Robert Collins (lifeless)]

+1 on printing stuff
-1 on a weakref dict, I rather suspect thats likely to trigger bugs and
gc edge cases, which our __del__'s already make tricky enough.

-Rob