Can I completely remove the history of all files in selected subdirs of my repo?

I have a testcase shell script.

I have a bloated repository, call it "oldrepo", that I'm trying to refactor. The repository contains several directories of large files that I'm not interested in keeping. 开发者_Python百科 I want them completely removed from history. I know how to remove specific files using filter-branch, but I don't know how to remove everything EXCEPT for a specific directory or set of directories without going through and removing every file individually.

I thought I could fool git into doing this by creating a new repo and only merging the files that I want to keep:

mkdir newrepo
git init; touch README; git add .; git commit -m "initial"
git remote add oldrepo /path/to/oldrepo
git merge -s ours --no-commit
git read-tree --prefix=subdir1 -u
git commit -m "merged subdir1"
git remote rm oldrepo
git prune --verbose

Unfortunately, the prune command prunes nothing. I was hoping that it would prune every object that had never been a child of SUBDIR1-TREE. Is there a way to remove all history outside of a directory without individually removing each offending file?

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This is also a job for filter-branch, but instead of removing select files/directories as mentioned in the other question you can empty out the index completely using git read-tree --empty (for Git v1.7.4 or newer; if you're running an older version of Git use git read-tree with no arguments) and then restore the files/directories you want using git reset:

git filter-branch -f --index-filter 'git read-tree --empty && git reset -q "${GIT_COMMIT}" -- first_directory_to_keep second_directory_to_keep'