Travis: Fix frozen tools check never failing

Having recently moved the frozen tools check to a standalone stage
at the end, we need to fetch the base branch so make comparison
work. Travis environments are not shared across different stages.

Without the fetch, the check always passes as a PR is compared with
itself.

.travis.yml

@@ -317,6 +317,9 @@ matrix:
       stage: "Frozen tools check"
       name: "Frozen tools check"
       env: NAME=frozen_tools_check
+      before_script:
+        # Fetch the base branch to compare against
+        - git fetch origin "${TRAVIS_BRANCH}" --depth=1
       script:
         # Reject any changes to tools that would require a re-release of the
         # tools for the online compiler.