[merge] Control-backspace to drop into the debugger

[Martin Pool]

>
> I should maybe explain them in HACKING as well.

I'd like to also add



Debugging
=========

Bazaar has a few facilities to help debug problems by going into pdb_, the
Python debugger.

http://docs.python.org/lib/debugger-commands.html

If the ``BZR_PDB`` environment variable is set
then bzr will go into pdb post-mortem mode when an unhandled exception
occurs.

If you send a SIGQUIT signal to bzr, which can be done by pressing C-\ on Unix,
bzr will go into the debugger immediately.  You can continue execution by
typing ``c``.





-- 
Martin