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`.
You can always and in any time check the difference between commits with

```
git diff commit1..commit2

```

You can substitute commit with branch names.

```
git diff master..foo

```

or also with HEAD

```
git diff master..HEAD

```

and so on.

Here a real example:

```
git diff e8e1956c174da9d..90f2b3ee988d8b6

```

Note that the complete hash is not necessary.