Travis clang-format, cpplint: Use the merge base

We previously looked at the diff to the current head of the target
branch, which may have moved in comparison to the merge base of a pull
request. Thus we sometimes ended up with spurious clang-format errors,
talking about changes introduced in another PR.

Author: tautschnig

.travis.yml

@@ -18,7 +18,7 @@ jobs:
         clang-format-7 --version
         # build a pathspec that excludes the files in .clang-format-ignore
         while read file ; do echo EXCLUDES+="':(top,exclude)$file' " ; done < .clang-format-ignore
-        git-clang-format-7 --binary clang-format-7 "${TRAVIS_BRANCH}" -- $EXCLUDES
+        git-clang-format-7 --binary clang-format-7 "${TRAVIS_COMMIT_RANGE%...*}" -- $EXCLUDES
         git diff > formatted.diff
         if [[ -s formatted.diff ]] ; then
           echo 'Formatting error! The following diff shows the required changes'

scripts/travis_lint.sh

@@ -12,5 +12,5 @@ else
   git config remote.origin.fetch +refs/heads/$TRAVIS_BRANCH:refs/remotes/origin/$TRAVIS_BRANCH
   git fetch --unshallow
   git checkout $TMP_HEAD
-  $script_folder/run_diff.sh CPPLINT origin/$TRAVIS_BRANCH # Check for errors compared to merge target
+  $script_folder/run_diff.sh CPPLINT ${TRAVIS_COMMIT_RANGE%...*} # Check for errors compared to merge base
 fi