Question: I have this:

```
branch: commit 1, commit 2, 3,4
master: commit 1, commit 2, 3,4,5,6,7

```

How can you make a diff between branch after `commit 2` (in that point of history) and master in `commit 3`?

Answer: 
Let's name your commits b1, b2, b3, b4; m1, m2, m3, m4, m5, m6, m7. To diff two commits you can run `git diff commit1 commit2`. In your case it's `git diff b2 m3`.
The detailed answer could be:

First checkout branch: git checkout branch

Then run: git log 

Get the SHA/commit-id of the commit you want and copy it. (I am calling it SHA1) 

Then go back to the master branch: git checkout master

Then run: git log 

Get the SHA/commit-id of the commit you want and copy it. (I am calling it SHA2)

Then while staying on Master, you can run: git diff SHA1 SHA2