How to move a file from one git repository to another while preserving history

This worked for me, but with a whole directory.

As shown here

~$ cd neu
~/neu$ git filter-branch --subdirectory-filter FooBar HEAD
~/neu$ git reset --hard
~/neu$ git remote rm origin
~/neu$ rm -r .git/refs/original/
~/neu$ git reflog expire --expire=now --all
~/neu$ git gc --aggressive
~/neu$ git prune
~/neu$ git remote add origin git://github.com/FooBar/neu.git

EDIT: For a single file:

Filter the directory first:

 git filter-branch --prune-empty --subdirectory-filter myDirectory -- --all

Filter the single file:

 git filter-branch -f --prune-empty --index-filter "git rm --cached --ignore-unmatch $(git ls-files | grep -v 'keepthisfile.txt')"

Do some cleanup:

 git reset --hard
 git gc --aggressive
 git prune

This should do it.