Merge git repo into branch of another repo

Updated with "real-life" commands:

Start from your repo directory, make sure your working copy is clean (no files changed, added or removed).

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Make a new branch:

git checkout -b <my-branch>

Add the secondary remote, then fetch it:

git remote add <repo-name> [email protected]:xxx/<repo-name>.git
git remote update

Merge one of their branches in your current branch:

git merge <repo-name>/<their-branch>

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If you don't know which <their-branch> you want, then go for master

If you are sure you want to accept all remote changes and avoid conflicts (overwrite yours) then you can specify -X theirs as option for git merge in the last step.

If you want to add it in a subdirectory then you should probably use git submodules

You can't merge a repository into a branch. You can merge a branch from another repository into a branch in your local repository. Assuming that you have two repositories, foo and bar both located in your current directory:

$ ls
foo bar

Change into the foo repository:

$ cd foo

Add the bar repository as a remote and fetch it:

$ git remote add bar ../bar
$ git remote update

Create a new branch baz in the foo repository based on whatever your current branch is:

$ git switch -c baz

Merge branch somebranch from the bar repository into the current branch:

$ git merge --allow-unrelated-histories bar/somebranch

(--allow-unrelated-histories is not required prior to git version 2.9)