We were working on a project that had its own git repository and commit history.
One month later the requirements pointed towards moving the project into an existing one with its own commit history. And one month later we had to move a specific folder from the repository into a new one.
Believe it or not the repositories were merged preserving their commit history in both cases.
tl;dr Clear process of how to merge Repository B into Repository A git repositories, merging everything as it is, into a new directory or only one directory, preserving the commit history.
Merge Repository B into Repository A
Following these steps you will merge Repository B into Repository A preserving the same directories and file structure and history from both repositories.
Step 1: Access the Repository A directory to start the merge.
$ cd repository_a
Step 2: Add the Repository B as a remote within the Repository A.
$ git remote add repository_b ~/path/to/repository_b
Step 3: Pull the remote into the master branch or whatever branch you want.
$ git pull repository_b master
Step 4: Remove the remote we just added so we don’t mess up in The Future™.
$ git remote rm repository_b
Profit: By now, Repository A should have directories and files from Repository B as well as the commit history. You can run git log to make sure.
Merge it into a new directory
With this method you will merge Repository B into Repository A within a new directory or an existing directory preserving the git history from both repositories.
Step 1: Access the Repository B directory to start the merge setup.
$ cd repository_b
Step 2: Create the a custom directory structure inside the Repository B.
$ mkdir repo_b
For example, if you want all the content from Repository B to be inside the src/repo_b directory within the Repository A, you will need to create the following directory.
$ mkdir src/repo_b
Step 3: Move the files you want or all of them inside the new directory.
$ git mv * src/repo_b/.
Step 4: Commit the change.
$ git add .
$ git commit -m 'Prepare repository_b to merge into repository_a.'
Continue: Follow the instructions above from the sections Merge it into a new directory or Merge Repository B into Repository A.
Merge only one directory
With this method you will merge a specific directory from Repository B into Repository A preserving the git history from both repositories.
Step 1: Access the Repository B directory to start the merge setup.
$ cd repository_b
Step 2: Filter out using filter-branch git command to exclude everything else and rewrite the history from the directory. Note that this command will rewrite all the git history from the Repository B so working in copy of the project is recommended.
$ git filter-branch --subdirectory-filter <my_folder> <branch>
Continue: Follow the instructions above from the sections Merge it into a new directory or Merge Repository B into Repository A.
