kf6-kwindowsystem: C-7 migration patch (ecm_install_po_files_as_qm) + script exclude

Second durable C-7 migration patch. Captures the inline
sed chain in kf6-kwindowsystem's [build].script that comments
out the `ecm_install_po_files_as_qm(poqm)` line in
CMakeLists.txt. The patch is a 16-line single-file edit
(CMakeLists.txt only) with no autogenerated noise.

Script change: the diff command now also excludes
`--exclude='.clang-format'` and `--exclude='.gitignore'`,
which ECM (Extra CMake Modules) writes on every cmake
configure step. Without these excludes, the patch would be
~120 lines of ECM autogenerated noise with the real
Red Bear edit buried in the middle. This is a regression
fix for kf6-kwindowsystem which had a clean real diff
hidden under 95+ lines of clang-format config.

Adds test_diff_excludes_ecm_generated_files (124 Python
tests total, 17 in this file). All 7 test files pass.

Migration status: 2/56 KF6 recipes migrated to external
patches (kf6-karchive, kf6-kwindowsystem). The remaining
54 recipes will be migrated as their cook+diff completes;
the migration script is now runnable end-to-end with
no manual filtering required.

local/scripts/migrate-kf6-seds-to-patches.sh

@@ -179,8 +179,16 @@ for recipe_dir in "${recipe_dirs[@]}"; do
     timeout 600 ./target/release/repo cook "$name" >>"$log_file" 2>&1 || true
 
     # Step 4: diff pristine vs post-cook.
+    #
+    # Exclude ECM-generated files that the cmake configure step
+    # creates on every run: `.clang-format` (clang-format style
+    # config), `.gitignore` (gitignore rules for the build dir),
+    # and any `target/` build outputs. These are NOT Red Bear
+    # edits and would pollute the patch with autogenerated noise.
     diff_out=$(diff -ruN "$pristine_dir" "$recipe_dir/source" \
-        --exclude='.git' --exclude='target' 2>/dev/null || true)
+        --exclude='.git' --exclude='target' \
+        --exclude='.clang-format' --exclude='.gitignore' \
+        2>/dev/null || true)
     if [ -z "$diff_out" ]; then
         echo "  NOTE: cook produced no diff (sed chains may have been no-ops)"
         rm -rf "$pristine_dir"

local/scripts/tests/test_migrate_kf6_seds.py

@@ -219,6 +219,25 @@ class TestScriptStructure(unittest.TestCase):
             "cook call must be wrapped in `timeout N`",
         )
 
+    def test_diff_excludes_ecm_generated_files(self):
+        # ECM (Extra CMake Modules) writes a `.clang-format`
+        # config file and a `.gitignore` file during the cmake
+        # configure step on every cook. These are NOT Red Bear
+        # edits and would pollute the migration patch with
+        # autogenerated noise (95+ lines for `.clang-format`
+        # alone). The diff must exclude them.
+        text = SCRIPT.read_text()
+        self.assertIn(
+            ".clang-format",
+            text,
+            "diff command must exclude .clang-format (ECM autogenerated)",
+        )
+        self.assertIn(
+            ".gitignore",
+            text,
+            "diff command must exclude .gitignore (ECM autogenerated)",
+        )
+
 
 if __name__ == "__main__":
     unittest.main()