This command, given a commit range commit1..commit2, rewrites history so that git commit author becomes also git committer:

git filter-branch -f --commit-filter \\
   'export GIT_COMMITTER_NAME=\\"$GIT_AUTHOR_NAME\\";
    export GIT_COMMITTER_EMAIL=\\"$GIT_AUTHOR_EMAIL\\";
    export GIT_COMMITTER_DATE=\\"$GIT_AUTHOR_DATE\\";
    git commit-tree $@' \\
    -- commit1..commit2