轉自  https://stackoverflow.com/questions/2428137/how-to-rebase-one-git-repository-onto-another-one

If A and B are not the same repo (you created B by using the latest working copy you had), you have to use a graft to pretend that they have common history.

Let’s assume you’ve added A as a remote for B as per

~/B$ git tnylog 
* 6506232 (HEAD, master) Latest work on B
* 799d6ae Imported backup from USB stick
~/B$ git tnylog A/master
* 33b5b16 (A/master) Head of A
* 6092517 Initial commit

Create a graft telling the root of B that its parent is the head of A:

echo '799d6aeb41095a8469d0a12167de8b45db02459c 33b5b16dde3af6f5592c2ca6a1a51d2e97357060' \
 >> .git/info/grafts

Now the two histories above will appear as one when you request the history for B. Making the graft permanent is a simple git filter-branch with no arguments. After the filter-branch, though, you aren’t on any branch, so you should git branch -D master; git checkout -b master

¹ git tnylog = git log --oneline --graph --decorate

補充句話：Git太厲害了！！！