Code review comment for lp://qastaging/~benoit.pierre/bzrtools/shell_improvements

Various improvements for the shell command:

- allow killing the current command line and start with a new one with ^C like
  most shells do (which also helps avoiding killing the shell by mistake when
  one's timing is off when trying to interrupt a command in progress), ^D can
  be used to exit when on an empty line

- catch shlex.split ValueError exceptions: the erroneous command can be edited
  again by using <up> to navigate the readline history

  (fix https://bugs.launchpad.net/bzrtools/+bug/231020)

- add global config aliases to the list of possible commands when completing

- fix shell handling of alias so for example "s|grep pattern" correctly works
  if s is an alias

- add wrapper for reconfigure/switch command so prompt gets updated to reflect
  the new tree

  (fix https://bugs.launchpad.net/bzrtools/+bug/231110)