How to split a git repository and follow directory renames?

Thanks to @Chronial, I was able to cook a script to massage my git repo according to my needs:

git filter-branch --prune-empty --index-filter '
    # Delete files which are NOT needed
    git ls-files -z | egrep -zv  "^(NAME1|NAME2|NAME3)" | 
        xargs -0 -r git rm --cached -q             
    # Move files to root directory
    git ls-files -s | sed -e "s-\t\(NAME1\|NAME2\|NAME3\)/-\t-" |
        GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
        git update-index --index-info &&
        ( test ! -f "$GIT_INDEX_FILE.new" \
            || mv -f "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE" )
'

Basically what this does is this:

1. Deletes all files outside of the three directories NAME1, NAME2 or NAME3 that I need (one project was renamed NAME1 -> NAME2 -> NAME3 during its lifetime).

2. Moves everything inside these three directories to the root of the repository.

3. I needed to test if “$GIT_INDEX_FILE.new” exists since import of svn into git creates commits without any files (directory-only commits). Needed only if the repo was created with ‘git svn clone’ initially.