`git diff-tree` used to fail on the `3.2` branch (and other non-master
branches) as Travis doesn't actually check that branch from the remote:
```
fatal: ambiguous argument '3.2': unknown revision or path not in the
working tree.
```
The exit code would still be 0 so we'd miss badly formatted commits
targeting stable branches.
We do it manually to ensure that it's going to work as we want it.
(cherry picked from commit e479231b21)
c071c754d0