How to: GIT: The parent of the root commit on one repository is the latest commit on the other. It is by awesome guidance from kevin known as _ikke_ in #git irc channel.

tutorial.txt

1. First clone both the repos. both repos live under their respective dir, project1 and project2. and they have their own .git dir.

 cd parentdir
parentdir.
|-- project1
|   |-- app
|   `-- .git
`-- project2
    |-- app
    `-- .git


2. Add project2 to project1 as a remote. If you want to project2 should be appended to project1. e.g project1 commits + project2 commits

cd project1
git remote add project2 path/to/project2/.git

3. git fetch project2

4. Both repos have a branch called master. (In my case it was).

5. git rev-list project2/master | tail -n1
   that should give you the hash of the first commit

6. git show <hash> 
   to confirm

7. git rev-parse master 
   get the hash of the last commit on the master branch

8. git show <hash>
   to confirm

9. echo "<root_hash> <master_hash>" >> .git/info/grafts
   where root_hash is the hash from step 5.
   and master_hash is from step 7

10. git log root_hash --oneline
    You should see project1's all commit + the first commit hash from project2
    So in a way there would be count(project1's commit)+1 number of commits

11. git filter-branch project2/master 
    To make the link permanent
    
12. git checkout -B master project2/master
    to reset master to the latest commit