Merge a separate Git repository into an existing one

When I started on a project it seemed to make sense to put a part of the project in a separate Git repository. In hindsight that wasn’t such a smart move. Here’s how I fixed it.

The old situation

In the old situation I had two Git repositories: <project> and <package>. In this case <project> was the project repository and <package> contained only one part of it. (For those interested: <package> is a Python package which I included into the project using mr.developer.) A simplified version of the situation looks like this:

├── buildout.cfg
└── src
    └── <package>

For several reasons I wanted to merge the <package> repository into the <project> repository in the src/package path.

There are several ways to approach this. I wanted to end up in a situation where in my day to day work I would not notice that the two repositories were separate up until a certain point.

What I did not do

At first I tried the approach outlined by Jason Karns in his article Merge Two Git Repositories Into One. That is, I did not create a new empty repository to merge the two existing repositories in. I just merged one existing repository into the other, essentially only doing the second set of steps he described.

After I finished, I discovered that I could not easily use “git log” to see the history of a file. Sure, I could use the “--follow” option but that only works for a single file—not a complete directory. Apparently this is caused by the “git read-tree” step. And although you can fix this, I wanted to avoid the situation.

In his article Merge Git Repositories and Preserve Commit History, Scott W. Bradley describes a way to do the merge without using the “git read-tree” command. However, the result is similar due to the “git mv” step that is in there.

The method I used

What I wanted apparently was a bit more complex. As a result the process is also a little more complex. Thankfully I could combine the previously mentioned articles with a helpful answer on Stack Overflow. This resulted in the following ‘recipe’:

First clone the <package> repository and go to that directory:

$ git clone ssh://<package-repo> /tmp/package
$ cd /tmp/package

Just to be sure we do not commit something in the original repo, remove the remote:

$ git remote rm origin

Then use “git filter-branch” to rewrite the existing commits so that the files are already in the right directory (src/package in my case):

$ git filter-branch --index-filter \
      'git ls-files -s | sed "s-\t\"*-&src\/package/-" |
       GIT_INDEX_FILE=$ \
       git update-index --index-info &&
       mv "$" "$GIT_INDEX_FILE"
      ' HEAD

(Note that according to Frederik you have to replace the \t in the sed command with Ctrl-V + tab when using OS X.)

You can now verify that everything is still all-right, the history is preserved and all files are located in the new directory.

Now make a fresh clone of the <project> repo:

$ git clone ssh://<project-repo> /tmp/project
$ cd /tmp/project

Add the <package> clone as a remote:

$ git remote add -f package /tmp/package

Next, merge the new remote:

$ git merge --allow-unrelated-histories package/master

(Updated on 2017-05-03 to add --allow-unrelated-histories, which is needed since Git 2.9. Thanks to Josef, Maurits and Duncan for pointing this out.)

Cleanup time: you can remove the temporary <package> remote:

$ git remote rm package

By now all code should be in the same place as it was before we started, but now it’s in a single repository. Now would be a nice time to run your tests to verify that everything went well.

If everything checks out, don’t forget to push the result to the <project> repository. (What you do with the <package> repository is up to you. I would probably remove it.)