Travis: Fix clang-format on non-master branches

`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.

0 files changed, 0 insertions, 0 deletions