Merge a Git Repo into Another Git Repo as a Branch

I have a "main" git repo that I use for most of our codebase (all of it, really). We recently begun an Android project, and had setup Android-Studio's VCS (version control system) incorrectly. About a dozen commits had been recorded in that system before we realized that we wanted to integrate it into our main repo.

I didn't want to use subtree or submodule, instead preferring to integrate it into a branch in the main repo. I also wanted to preserve whatever commit history was already in the VCS repo.

Here is how I did it...
(NOTE: make sure you're first checked-out in the new branch that you want to import/merge the VCS repo into, before continuing)

First, I went into the main repo (just using the git command line) and added the VCS repo as a remote:
# git remote add -f <remote-name> ssh://<user>@<remote-host>/path/to/androidStudio/asProject

Then, I did a fetch of that remote repo (the one with the Android project) into the main repo:
# git fetch <remote-name>

Next, I did a manual merge with the "ours" strategy (VCS just had one branch -master- in my case):
# git merge -s ours --no-commit <remote-name>/master

Next, you have to read all the stuff in:
(NOTE: this is where you can specify what subdirectory you want the "imported" repo to reside)
# git read-tree --prefix <main-repo/dest-path/asProj/> -u <remote-name>/master

Finally, you are ready to commit your the Android VCS repo into your main repo:
(you may do a status first, to double-check things before committing)
# git status
# git commit -m "adding existing Android Studio VCS repo to this new branch in main repo"

That's it!
To view your VCS commit history, just be sure the remote is visible in whatever GUI you use.

If you want to update the main repo with any changes in the Android Studio VCS repo (since you probably want to use that IDE in that environment), just pull it in with the "subtree" strategy:
# git pull -s subtree <remote-name> master