Create a Git repo with only the history and content of /blueprint

Originally created by @sajolida on #9175 (Redmine)

This involves creating a new Git repo that:

1. Only contains the blueprint directorie’s content.
2. Inherits the current Git repo’s history for that content, and nothing else.

git-filter-branch(1) should be a good start.

To rewrite the repository to look as if foodir/ had been its project root, and discard all other history:

git filter-branch --subdirectory-filter foodir -- --all

Edited May 15, 2020 by sajolida