I want to prefix all files inside the index file. I could achieve it using this git command:

TAB=$'\t' git filter-branch --index-filter '
  git ls-files -s |
  sed "s-$TAB\"*-&new_prefix/-" |
  GIT_INDEX_FILE=$GIT_INDEX_FILE.new git update-index --index-info &&
  mv $GIT_INDEX_FILE.new $GIT_INDEX_FILE
' HEAD

What would be an approach that I could use for libgit2? Is the goto method to iterate over each index entry, changing the file path the entry points to and adding the entry back to the index?