Merging Related Git Repositories

First, make the legacy repo a remote of your repo. It can be referenced by a URL or by a file path.

git remote add legacy <path or URL to legacy>

Fetch it.

git fetch legacy

A [legacy/master]
|
B        F [master]
|        |
C        G
|        |
D        H
|
E

At this point we could smack them together with git merge --allow-unrelated-histories but that increases the chance of a bad merge. Instead, first stitch the histories together by rewriting the commits in your new repo onto the commit where they diverged. Let’s say that was commit D.

git rebase D

[legacy/master] A
                |  F1 [master]
                B  |
                |  G1
                C /
                |/
                D
                |
                E

Note that the first commit in the new repo should have the same content as D and will probably disappear. If it remains, it should only contain the differences introduced by the copy. These may be significant.

Now merge normally.

git merge legacy/master

                   M [master]
                  /|
                 / |
[legacy/master] A  |
                |  F1
                B  |
                |  G1
                C /
                |/
                D
                |
                E

And make this the central repo going forward.