diff --git a/build-scripts/build.sh b/build-scripts/build.sh
index 573e7a4d74c9d..008794662c3d4 100755
--- a/build-scripts/build.sh
+++ b/build-scripts/build.sh
@@ -130,14 +130,17 @@ then
         cd ..
         ln -s build/compile_commands.json
 
+        ./build-scripts/files_changed
+
         # We want to first analyze all files that changed in this PR, then as
         # many others as possible, in a random order.
         set +x
         all_cpp_files="$( \
             grep '"file": "' build/compile_commands.json | \
             sed "s+.*$PWD/++;s+\"$++")"
+        changed_files="$( ./build-scripts/files_changed || echo unknown )"
         changed_cpp_files="$( \
-            ./build-scripts/files_changed | grep -F "$all_cpp_files" || true )"
+            echo "$changed_files" | grep -F "$all_cpp_files" || true )"
         if [ -n "$changed_cpp_files" ]
         then
             remaining_cpp_files="$( \
@@ -160,8 +163,15 @@ then
         echo "Analyzing changed files"
         analyze_files_in_random_order "$changed_cpp_files"
 
-        echo "Analyzing remaining files"
-        analyze_files_in_random_order "$remaining_cpp_files"
+        # Check for changes to any files that would require us to run clang-tidy across everything
+        changed_global_files="$( \
+            echo "$changed_files" | egrep -i "\.h$|clang-tidy-plugin|cmake|unknown" || true )"
+        if [ -n "$changed_global_files" ]
+        then
+            first_changed_file="$(echo "$changed_global_files" | head -n 1)"
+            echo "Analyzing remaining files because $first_changed_file was changed"
+            analyze_files_in_random_order "$remaining_cpp_files"
+        fi
         set -x
     else
         # Regular build
diff --git a/build-scripts/files_changed b/build-scripts/files_changed
index 27bbd9ef097be..b522337362f13 100755
--- a/build-scripts/files_changed
+++ b/build-scripts/files_changed
@@ -3,15 +3,39 @@
 # Helper script to get the list of changed files for this build.
 # If a PR it lists the changes from the target branch.
 # If a 'regular' branch build the changes since the last build.
-# https://docs.travis-ci.com/user/environment-variables/
 
-if [ -n $TRAVIS_COMMIT_RANGE ]
+set -e
+
+function files_list
+{
+    git diff --name-only --diff-filter ACMRT "$@"
+}
+
+if [ "$TRAVIS" = "true" ]
+then
+    # https://docs.travis-ci.com/user/environment-variables/
+    if [ -n $TRAVIS_COMMIT_RANGE ]
+    then
+        # If this string is populated, it should work.
+        # But it mught fail, due to e.g. the shallow clone depth missing the
+        # relevant commit, in which case we fail
+        files_list $TRAVIS_COMMIT_RANGE || exit 1
+    else
+        # The only time it isn't populated is on a new PR branch, where THIS will work.
+        files_list $TRAVIS_BRANCH
+    fi
+elif [ -n "$GITHUB_BASE_REF" ]
 then
-    # If this string is populated, it should work.
-    # But it mught fail, due to e.g. the shallow clone depth missing the
-    # relevant commit, in which case we fail
-    git diff --name-only $TRAVIS_COMMIT_RANGE || exit 1
+    base="origin/$GITHUB_BASE_REF"
+    # We need to fetch enough commits to be able to compute the merge-base
+    # between this PR and master
+    commits_to_fetch=10
+    while [ -z "$( git merge-base "$base" HEAD^2 2>/dev/null )" ]
+    do
+        git fetch -q --deepen=$commits_to_fetch origin "$GITHUB_BASE_REF" HEAD
+        (( commits_to_fetch *= 2 ))
+    done
+    files_list "$( git merge-base "$base" HEAD^2 )..HEAD^2"
 else
-    # The only time it isn't populated is on a new PR branch, where THIS will work.
-    git diff --name-only $TRAVIS_BRANCH
+    exit 1
 fi