Move one git repository into another as a subdirectory without losing the commit histories.

move_one_git_repo_into_another_as_a_subdir

# Let's say we have two repositories repo_A and repo_B.
# And following are the directory structure of both repositories,
# repo_A
#      | - backend
#      | - index.js
# repo_B
#      | - components
#      | - js
#      | - index.js

# Now the requirement is to move repo_B into repo_A as a subdirectory without losing the commit histories.

# First, clone both the repos if you don't have it already in your local.
git clone repo_A
git clone repo_B

# Go to repo_B and create a subdirectory as you want it in repo_A (let's call it frontend).
cd repo_B
mkdir frontend

# Move all the files and folders into the subdirectory created.
git mv components frontend/
git mv js frontend/
git mv index.js frontend/

# Commit the changes made.
git commit -m "Move all the files and directories into subdirectory"

# Now, the directory structure of repo_B looks as follows, 
# repo_B
#      | - frontend
#                 | - components
#                 | - js
#                 | - index.js

# Now, go to repo_A and add a new remote to repo_B
cd ../repo_A
git remote add repo_B ../repo_B
git fetch repo_B

# Check if the new remote is added properly using the following command,
git remote -v

# Merge the remote created.
git merge repo_B/master --allow-unrelated-histories

# Now, you can remove the remote created.
git remote rm repo_B

# After the merge, the directory structure of repo_A will look like,
# repo_A
#      | - backend
#      | - frontend
#                 | - components
#                 | - js
#                 | - index.js
#      | - index.js

# Thanks :)