.bzrignore should support exclusions

There are compelling cases for supporting ignore exclusions in the .bzrignore format, including versioning of "live" directories such as the rcfiles within a user's home directory, or system configuration files (e.g. files under /etc). Exclusions also improve utility of the ignore files for cases where a mix of auto-generated (and ignored) files coexist with hand-maintained files, such as generated vs. hand-coded HTML files.

I propose to mimic git's syntax for ignore exclusions, which is that prepending an exclamation point (!) to a line in .bzrignore explicitly indicates that matching files be considered trackable.

Example, a .bzrignore file for versioning rcfiles:
----------------
# Ignore everything except files and directories covered below.
*
# We're using zsh, so track all files and direcories starting with '.z'.
# This covers the standards such as .zshrc, .zaliases, etc. and
# files (.zglobalfns) and dirs (.zfunctions/) custom to this project
.z**
# Also version ~/local
~/local
~/local/**
----------------

I haven't investigated the current ignore semantics in bzrlib in detail -- the above example might change a bit depending on the particulars. For example, are both the "~/local" and "~/local/**" lines necessary for a simple implementation of exclusions? Should they be (or not)?