Git merge submodule into parent tree cleanly and preserving commit history

Question

I have a repository with two submodules that I want to convert into a single project. Many answers involve scripts, and some seem to be overcomplicated.

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Answer 1

A bit late to the party, but for those still looking for help, checkout these:

1. https://gitirc.eu/howto/using-merge-subtree.html
2. https://help.github.com/en/github/using-git/about-git-subtree-merges

Below is a somewhat verbatim copy the first post:

1. git remote add -f Bproject /path/to/B
2. git merge -s ours --no-commit --allow-unrelated-histories Bproject/master
3. git read-tree --prefix=dir-B/ -u Bproject/master
4. git commit -m "Merge B project as our subdirectory"

5. git pull -s subtree Bproject master

The explanation

1. name the other project "Bproject", and fetch.
2. prepare for the later step to record the result as a merge.
3. read "master" branch of Bproject to the subdirectory "dir-B".
4. record the merge result.

5. pull in subsequent update from Bproject using "subtree" merge

As an alternative before step 4, you might want to update .gitmodules file, or just remove it:

3.1 git rm --cached .gitmodules

With this the history from the submodule is well preserved.