How to extract one file with commit history from a Git repo with index-filter & co?

I have a Git repo converted from SVN to Mercurial to Git, and I wanted to extract just one source file. I also had weird characters like aÌ (an encoding mismatch corrupted Unicode ä) and spaces in the filenames.

How can I extract one file from a repository and place it at the root of the new repo?

Solution

There is a new command git filter-repo nowadays. It has more possibilities and better performance.

See man page for details and project page for installation.

Remove everything except src/README.md and move it to the root:

git filter-repo --path src/README.md
git filter-repo --subdirectory-filter src/

--path selects the single file and --subdirectory-filter moves the contents of that directory to root.