reference: http://stackoverflow.com/a/750191
be careful if you have a multi-user repository - this will change ALL commits to have the same (new) author and committer.
#!/bin/sh
git filter-branch -f --env-filter "
GIT_AUTHOR_NAME='loivis'
GIT_AUTHOR_EMAIL='[email protected]'
GIT_COMMITTER_NAME='loivis'
GIT_COMMITTER_EMAIL='[email protected]'
" HEAD