Today I went to #debian-x and asked how they have been doing to update debian/changelog files and they said that they're using GIT log but updating the files by-hand so, since we're wanting to use it here at work, I've started to write up the script for it.
After some time of hacking and some questions answered at #git, on freenode, I was able to get it working. Obviously it has to be improved (hey, send me a patch!) but it works for my tests.
It has been made availabe at http://git.debian.org/?p=users/otavio/git-update-debian-changelog.git
Update: A new version has been made available since we' ve found two minor bugs on it.
Update: Support to --snapshot has been included.