How do I merge a sub directory in Git?

Just as an alternative to the SO question “How do you merge selective files with git-merge?”, I just found this GitHub thread which could be more adapted for merging a whole subdirectory, based on git read-tree:

• My repository => cookbooks
  My repository target directory => cookbooks/cassandra

• Remote repository => infochimps
  Remote repository source I want merged into cookbooks/cassandra => infochimps/cookbooks/cassandra

Here are the commands I used to merge them

• Add the repository and fetch it

  git remote add -f infochimps git://github.com/infochimps cluster_chef.git
  

• Perform the merge

  git merge --allow-unrelated-histories -s ours --no-commit infochimps/master
  

(this performs a merge by using the ‘ours’ strategy (-s ours), which discards changes from the source branch.
This records the fact that infochimps/master has been merged, without actually modifying any file in the target branch)

• Merge only infochimps/cookbooks/cassandra into cassandra

  git read-tree --prefix=cassandra/ -u infochimps/master:cookbooks/cassandra
  

This reads the tree for only the required source subdirectory i.e. cookbooks/cassandra, on the upstream branch of the source repository.

Note that the target subdirectory name should also be cookbooks/cassandra, or you would see:

fatal: Not a valid object name

• Commit the change

  git commit -m 'merging in infochimps cassandra'
  

Addendum

It’s bizarre,^{[edit me]} — but the read-tree step can possibly fail like this:

error: Entry 'infochimps/cookbooks/cassandra/README' overlaps with 'cookbooks/cassandra/README'. Cannot bind.

… even when both files are identical. This might help:

git rm -r cassandra
git read-tree --prefix=cassandra/ -u infochimps/master:cookbooks/cassandra

But off course, verify manually that this does what you want.