How to split a git repository while preserving subdirectories?

Question

What I want is similar to this question. However, I want the directory that is split into a separate repo to remain a subdirectory in that repo:

I have this:

Answer 1

A cleaner method:

git filter-branch --index-filter '
                git read-tree --empty
                git reset $GIT_COMMIT path/to/dir
        ' \
        -- --all -- path/to/dir

or to stick with just core commands, sub in git read-tree --prefix=path/to/dir/ $GIT_COMMIT:path/to/dir for the reset.

Specifying path/to/dir on the rev-list args does the pruning early, with a filter this cheap it doesn't matter much but it's good to avoid the wasted effort anyway.