Minimize clang-tidy CI runtime (CleverRaven#50131)

clang-tidy is one of our slowest CI jobs.  It would be nice if it were
faster.

When only cpp files are changed, we can limit the checks to just those
files.  When a header or build file is changed, or a new check is added,
we need to recheck everything.

To make this work, update the logic in build.sh which previously
prioritised changed files.  Make it now only check changed files,
provided that's appropriate.

Also, make files_changed work correctly on GitHub, at least for PR jobs.
It will simply fail on non-PR jobs.

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

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