We should be more realistic in our output. Please, take a look on this real diff header:
```
diff --git a/README b/README
index bccdfbd..b0ed415
```
Here ```a``` and ```b``` - marks to differ changes from commits; ```README``` - file name
```bccdfbd..b0ed415``` - base and head hashes.
Let's generate output of that format and substitute revision hashes. File name can be ```README```, as we know nothing about file name in mock