CMake: Fix travis fetch PR (#13763)

Either fetch TRAVIS_COMMIT or the entire pull request (depending on the TRAVIS_PULL_REQUEST environment variable).

.travis.yml

@@ -134,8 +134,10 @@ matrix:
         - >-
           git clone --depth=1 --single-branch https://github.com/ARMmbed/mbed-os.git;
           cd mbed-os;
-          git fetch origin +refs/pull/${TRAVIS_PULL_REQUEST}/merge;
-          git checkout -qf FETCH_HEAD
+        - to_fetch=("${TRAVIS_COMMIT}")
+        - if [ false != "${TRAVIS_PULL_REQUEST-}" ]; then to_fetch+=("+refs/pull/${TRAVIS_PULL_REQUEST}/merge:"); fi
+        - git fetch -q -- origin "${to_fetch[@]}"
+        - git checkout -qf "${TRAVIS_COMMIT}" --
         - >-
           cd -;
         # Replace the content of mbed-os.lib to stay on ${TRAVIS_PULL_REQUEST}