Corrected simplified astyle job behavior when base branch is not master

Consolidated efforts from PR ARMmbed#9336.

Previous job's env vars would not be populated correctly if the base branch of a PR was not master.
Corected by pulling remaining respository information to perform comparison between read-only instance of PR and base branch.

Author: Cruz Monrreal II

.travis.yml

@@ -144,9 +144,12 @@ matrix:
           export PATH=$PWD/bin:$PATH;
           cd -
         - astyle --version
+        # Fetch remaining information needed for branch comparison
+        - git fetch --all --unshallow --tags
+        - git fetch origin "${TRAVIS_BRANCH}"
       script:
         - >-
-          git diff --name-only --diff-filter=d HEAD..${TRAVIS_BRANCH} \
+          git diff --name-only --diff-filter=d FETCH_HEAD..HEAD \
             | ( grep '.\(c\|cpp\|h\|hpp\)$' || true ) \
             | ( fgrep -v -f .astyleignore || true ) \
             | while read file; do astyle -n --options=.astylerc "${file}"; done