Add Kokoro test filtering for MacOS/Windows

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -32,6 +32,13 @@ ulimit -a
 sudo easy_install --upgrade google-api-python-client
 export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/GrpcTesting-d0eeee2db331.json
 
+# If this is a PR using RUN_TESTS_FLAGS var, then add flags to filter tests
+if [ -n "$KOKORO_GITHUB_PULL_REQUEST_NUMBER" ] && [ -n "$RUN_TESTS_FLAGS" ]; then
+  brew install jq
+  ghprbTargetBranch=$(curl -s https://api.github.com/repos/grpc/grpc/pulls/$KOKORO_GITHUB_PULL_REQUEST_NUMBER | jq -r .base.ref)
+  export RUN_TESTS_FLAGS="$RUN_TESTS_FLAGS --filter_pr_tests --base_branch origin/$ghprbTargetBranch"
+fi
+
 # required to build protobuf
 brew install gflags
 

tools/internal_ci/helper_scripts/prepare_build_windows.bat

@@ -18,6 +18,14 @@ set PATH=C:\tools\msys64\usr\bin;C:\Python27;%PATH%
 
 bash tools/internal_ci/helper_scripts/gen_report_index.sh
 
+@rem If this is a PR using RUN_TESTS_FLAGS var, then add flags to filter tests
+if defined KOKORO_GITHUB_PULL_REQUEST_NUMBER if defined RUN_TESTS_FLAGS (
+  chocolatey install -y jq
+  for /f "usebackq delims=" %%x in (`curl -s https://api.github.com/repos/grpc/grpc/pulls/%KOKORO_GITHUB_PULL_REQUEST_NUMBER% ^| jq -r .base.ref`) do ( 
+    set RUN_TESTS_FLAGS=%RUN_TESTS_FLAGS% --filter_pr_tests --base_branch origin/%%x
+  )
+)
+
 @rem Update DNS settings to:
 @rem 1. allow resolving metadata.google.internal hostname
 @rem 2. make fetching default GCE credential by oauth2client work

tools/run_tests/python_utils/filter_pull_request_tests.py

@@ -128,7 +128,7 @@ def _get_changed_files(base_branch):
   # Get file changes between branch and merge-base of specified branch
   # Not combined to be Windows friendly
   base_commit = check_output(["git", "merge-base", base_branch, "HEAD"]).rstrip()
-  return check_output(["git", "diff", base_commit, "--name-only"]).splitlines()
+  return check_output(["git", "diff", base_commit, "--name-only", "HEAD"]).splitlines()
 
 
 def _can_skip_tests(file_names, triggers):