bazel-diff: Print skipped targets (#1620)

Progress towards #1605
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 4a8108a..47c01c5 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2179,8 +2179,18 @@
     finally:
         shutil.rmtree(tmpdir)
 
-    filtered_targets = list(set(expanded_test_targets).intersection(affected_targets))
-    if len(filtered_targets) < len(expanded_test_targets):
+    config_target_set = set(expanded_test_targets)
+    remaining_targets = list(config_target_set.intersection(affected_targets))
+    if len(remaining_targets) < len(expanded_test_targets):
+        print_collapsed_group(
+            ":scissors: Successfully reduced test targets from {} to {}".format(
+                len(expanded_test_targets), len(remaining_targets)
+            )
+        )
+
+        skipped_targets = sorted(config_target_set.difference(remaining_targets))
+        eprint("Skipped targets:\n\t{}".format("\n\t".join(skipped_targets)))
+
         execute_command(
             [
                 "buildkite-agent",
@@ -2191,7 +2201,7 @@
             ]
         )
 
-    return filtered_targets
+    return remaining_targets
 
 
 def resolve_diffbase(diffbase):