Travis CI uses wrong changeset to build

```
(master) _____ ____________ update text in file A _____
\
\___ add file "C" ______
(branch1)
```

Expected changeset for `branch1` should be `add file "C"`
Currently the changeset is `add file "C"` && `update text in file A`

It appears that we currently compare the current master branch with the branch1.
What we want to do is, compare `branch1` with master at the time the branch diverted from master.

We currently use this snippet to calculate the changeset.
https://github.com/AppImage/appimage.github.io/blob/f3da57cbd6b35478deaa3b412ebd2cfdd74ae0c5/.travis.yml#L29

If I'm not mistaken `git log master..HEAD -p data` should calculate the changeset for the current branch.