As I mentioned earlier, we're transitioning from subversion to git, and one of the things we need to do is convert from many-projects-per-repository, which is what we had in subversion, to one-project-per-repository for git. This is partly to make things nicer, and partly because it will make Jenkins work better. So, thinking in subversion-terms, I thought I'd have to do it myself, and even tried by renaming all the files in one of the projects into the upper-level directory, removing all the other projects, committing locally, and then pushing to a new repo. Royal pain.

And then, I came across this. Oh. My. God. Git actually does the work for you. It's so easy, you just do the git filter-branch command, and suddenly your project is its own repository! Then, you just change the remote repository (git remote rm origin followed by git remote add new-url), and Bob's your uncle, you have a new, smaller repository.

I mean, seriously? That is AWESOME!

So, to be explicit, here's the series of commands:

    $ git clone repo-url
    $ cd repo
    $ git filter-branch --prune-empty --subdirectory-filter subdir-name master
    $ git remote rm origin
    $ git remote add new-repo-url
    $ git push -u origin --all --tags

Updated 05/12/2013
Okay, after messing around with this for some time, I've discovered that the above doesn't work quite right, because the pathnames in the commits are wrong - instead of being subdir-name/src/foo/Bar.java, it ends up being src/foo/Bar.java, which doesn't work. So, instead of trying to pull each subdirectory into their own repository, I'm taking the existing repository, with a bunch of subdirs, including the ones I'm interested in, and simply using git filter-branch to get rid of the ones I'm not interested in, as well as update the tags. So, here's what I've got now, which seems to work well:

    # Get a list of the directories I'm not interested in
    $ excludes=`echo $* | tr " " "|"`
    $ files=`ls | egrep -v $excludes | tr "\\n" " "`
    # Disconnect from remote for safety
    $ git remote rm origin
    # Get rid of all tags that don't fit my pattern
    $ git tag -l | grep -v $pattern | xargs git tag -d
    # Prune the directories I'm not interested in
    $ git filter-branch --tag-name-filter cat --prune-empty \
        --tree-filter "rm -rf ${files}" -- --all
    # Reset the indexes
    $ git reset --hard
    # Get rid of obsolete references in the logs
    $ git for-each-ref --format="%(refname)" refs/original/ | \
        xargs -n 1 git update-ref -d
    $ git reflog expire --expire=now --all
    $ git gc --aggressive --prune=now