Travis: Use shallow fetches

We only compare files in current PR with the base branch. There's no
need to fetch the full git history or branches that are not involved
in the comparison.

This is to save time in Travis runs.

.travis.yml

@@ -68,9 +68,8 @@ matrix:
       before_script:
         - mkdir -p SCANCODE
         - mkdir -p SCANCODE_NEW_FILES
-        # Fetch remaining information needed for branch comparison
-        - git fetch --all --unshallow --tags
-        - git fetch origin "${TRAVIS_BRANCH}"
+        # Fetch the base branch to compare against
+        - git fetch origin "${TRAVIS_BRANCH}" --depth=1
       script:
         # scancode does not support list of files, only one file or directory
         # we use SCANCODE directory for all changed files (their copies with full tree)
@@ -130,9 +129,8 @@ matrix:
           export PATH="${PWD}/bin:${PATH}";
           cd -
         - astyle --version
-        # Fetch remaining information needed for branch comparison
-        - git fetch --all --unshallow --tags
-        - git fetch origin "${TRAVIS_BRANCH}"
+        # Fetch the base branch to compare against
+        - git fetch origin "${TRAVIS_BRANCH}" --depth=1
       script:
         - >-
           git diff --name-only --diff-filter=d FETCH_HEAD..HEAD \
@@ -246,9 +244,8 @@ matrix:
         - python -m pip install --upgrade setuptools==40.4.3
         - pip install tabulate argparse
         - pip list --verbose
-        # Fetch remaining information needed for branch comparison
-        - git fetch --all --unshallow --tags
-        - git fetch origin "${TRAVIS_BRANCH}"
+        # Fetch the base branch to compare against
+        - git fetch origin "${TRAVIS_BRANCH}" --depth=1
       script:
         - >-
           git diff --name-only --diff-filter=d FETCH_HEAD..HEAD \