Update last entry if distribution was UNRELEASED rather than adding a new one

Imported from Debian bug 515921:

Package: bzr-builddeb
Version: 0.95
Severity: wishlist

Now that bzr-builddeb supports automatically updating the changelog, it
would be nice if it could update existing changelog entries if they are
for unreleased versions (by looking for UNRELEASED as distribution
name), rather than creating new versions.

I can't find any dch options to do this though, perhaps this can be done
through the ChangeLog class in python-debian ?

-- System Information:
Debian Release: 5.0
  APT prefers oldstable
  APT policy: (500, 'oldstable'), (500, 'unstable'), (500, 'stable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 2.6.26-1-openvz-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages bzr-builddeb depends on:
ii bzr 1.12-1 easy to use distributed version co
ii bzrtools 1.12.0-1 Collection of tools for bzr
ii devscripts 2.10.46 scripts to make the life of a Debi
ii dpkg-dev 1.14.25 Debian package development tools
ii fakeroot 1.12.1 Gives a fake root environment
ii patchutils 0.3.1-1 Utilities to work with patches
ii python 2.5.2-3 An interactive high-level object-o
ii python-apt 0.7.9~exp2 Python interface to libapt-pkg
ii python-central 0.6.8 register and build utility for Pyt
ii python-debian 0.1.12 Python modules to work with Debian

bzr-builddeb recommends no packages.

Versions of packages bzr-builddeb suggests:
ii bzr-svn 0.5.1-1 Bazaar plugin providing Subversion

-- no debconf information