How to move files from one git repo to another (not a clone), preserving history

If your history is sane, you can take the commits out as patch and apply them in the new repository:

cd repository
git log --pretty=email --patch-with-stat --reverse --full-index --binary -- path/to/file_or_folder > patch
cd ../another_repository
git am --committer-date-is-author-date < ../repository/patch 

Or in one line

git log --pretty=email --patch-with-stat --reverse -- path/to/file_or_folder | (cd /path/to/new_repository && git am --committer-date-is-author-date)

(Taken from Exherbo’s docs)

Having tried various approaches to move a file or folder from one Git repository to another, the only one which seems to work reliably is outlined below.

It involves cloning the repository you want to move the file or folder from, moving that file or folder to the root, rewriting Git history, cloning the target repository and pulling the file or folder with history directly into this target repository.

Stage One

1. Make a copy of repository A as the following steps make major changes to this copy which you should not push!

   git clone --branch <branch> --origin origin --progress \
     -v <git repository A url>
   # eg. git clone --branch master --origin origin --progress \
   #   -v https://username@giturl/scm/projects/myprojects.git
   # (assuming myprojects is the repository you want to copy from)
   

2. cd into it

   cd <git repository A directory>
   #  eg. cd /c/Working/GIT/myprojects
   

3. Delete the link to the original repository to avoid accidentally making any remote changes (eg. by pushing)

   git remote rm origin
   

4. Go through your history and files, removing anything that is not in directory 1. The result is the contents of directory 1 spewed out into to the base of repository A.

   git filter-branch --subdirectory-filter <directory> -- --all
   # eg. git filter-branch --subdirectory-filter subfolder1/subfolder2/FOLDER_TO_KEEP -- --all
   

5. For single file move only: go through what's left and remove everything except the desired file. (You may need to delete files you don't want with the same name and commit.)

   git filter-branch -f --index-filter \
   'git ls-files -s | grep $'\t'FILE_TO_KEEP$ |
   GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
   git update-index --index-info && \
   mv $GIT_INDEX_FILE.new $GIT_INDEX_FILE || echo "Nothing to do"' --prune-empty -- --all
   # eg. FILE_TO_KEEP = pom.xml to keep only the pom.xml file from FOLDER_TO_KEEP
   

Stage Two

1. Cleanup step

   git reset --hard
   

2. Cleanup step

   git gc --aggressive
   

3. Cleanup step

   git prune
   

You may want to import these files into repository B within a directory not the root:

1. Make that directory

   mkdir <base directory>             eg. mkdir FOLDER_TO_KEEP
   

2. Move files into that directory

   git mv * <base directory>          eg. git mv * FOLDER_TO_KEEP
   

3. Add files to that directory

   git add .
   

4. Commit your changes and we’re ready to merge these files into the new repository

   git commit
   

Stage Three

1. Make a copy of repository B if you don’t have one already

   git clone <git repository B url>
   # eg. git clone https://username@giturl/scm/projects/FOLDER_TO_KEEP.git
   

   (assuming FOLDER_TO_KEEP is the name of the new repository you are copying to)

2. cd into it

   cd <git repository B directory>
   #  eg. cd /c/Working/GIT/FOLDER_TO_KEEP
   

3. Create a remote connection to repository A as a branch in repository B

   git remote add repo-A-branch <git repository A directory>
   # (repo-A-branch can be anything - it's just an arbitrary name)
   
   # eg. git remote add repo-A-branch /c/Working/GIT/myprojects
   

4. Pull from this branch (containing only the directory you want to move) into repository B.

   git pull repo-A-branch master --allow-unrelated-histories
   

   The pull copies both files and history. Note: You can use a merge instead of a pull, but pull works better.

5. Finally, you probably want to clean up a bit by removing the remote connection to repository A

   git remote rm repo-A-branch
   

6. Push and you’re all set.

   git push
   

Yep, hitting on the --subdirectory-filter of filter-branch was key. The fact that you used it essentially proves there's no easier way - you had no choice but to rewrite history, since you wanted to end up with only a (renamed) subset of the files, and this by definition changes the hashes. Since none of the standard commands (e.g. pull) rewrite history, there's no way you could use them to accomplish this.

You could refine the details, of course - some of your cloning and branching wasn't strictly necessary - but the overall approach is good! It's a shame it's complicated, but of course, the point of git isn't to make it easy to rewrite history.