travis: Allow travis builds to access target branches

Because travis builds only fetch a single branch (master) by default,
test-commit-message.sh only had access to commits in the master branch.
In order to fetch the correct branch for our build, we need to run
'git config remote.origin.fetch..' with the target branch's information
before executing git fetch on the repo in before_install.

Now git will always fetch the appropriate branch.

.travis.yml

@@ -8,6 +8,7 @@ sudo: true
 dist: trusty
 before_install:
   - sudo apt update
+  - git config remote.origin.fetch +refs/heads/${TRAVIS_BRANCH}:refs/remotes/origin/${TRAVIS_BRANCH}
   - git fetch --unshallow
 install: 'make deps'
 script: 'make test'