toplevel: Show toplevel-unnecessary-branch only if branch != stable,beta

If branch: stable or default-branch: stable is set this is harmless
and matches the expect ref branch Flathub sets, so there is no point
in erroring on this.

Instead error if unexpected branch names are set.

No error is shown if the properties are missing

flatpak_builder_lint/checks/toplevel.py

@@ -24,7 +24,12 @@ def check_manifest(self, manifest: dict) -> None:
 
             branch = manifest.get("branch")
             def_branch = manifest.get("default-branch")
-            if branch in ("stable", "master") or def_branch in ("stable", "master"):
+
+            if branch not in ("stable", "beta", None) or def_branch not in (
+                "stable",
+                "beta",
+                None,
+            ):
                 self.errors.add("toplevel-unnecessary-branch")
                 self.info.add(
                     "toplevel-unnecessary-branch: Please remove the toplevel"

tests/test_manifest.py

@@ -89,7 +89,6 @@ def test_manifest_toplevel() -> None:
         "toplevel-no-command",
         "toplevel-cleanup-debug",
         "toplevel-no-modules",
-        "toplevel-unnecessary-branch",
     }
 
     not_founds = {
@@ -101,6 +100,7 @@ def test_manifest_toplevel() -> None:
     found_errors = set(ret["errors"])
 
     assert errors.issubset(found_errors)
+    assert "toplevel-unnecessary-branch" not in found_errors
 
     ret = run_checks("tests/manifests/base_app.json")
     found_errors = set(ret["errors"])