test: assert only template-rendered files are visible to fileglob during update

Whether files added by the user in the destination are visible to
filesystem filters during an update depends on whether the update
algorithm renders into the user-specified destination, which is an
implementation detail that may change (copier-org#2376). Assert only the
invariant shared by both algorithms: rendering happens in the
destination being rendered, never the invocation directory.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: nikzart

tests/test_updatediff.py

@@ -596,7 +596,7 @@ def test_update_render_cwd_is_destination(
         git_init("v1")
         git("tag", "v1")
     run_copy(str(src), dst, defaults=True, overwrite=True)
-    build_file_tree({(dst / "a.txt"): "", (invocation / "decoy.txt"): ""})
+    build_file_tree({(invocation / "decoy.txt"): ""})
     with local.cwd(dst):
         git_init("copied")
     with local.cwd(src):
@@ -608,7 +608,13 @@ def test_update_render_cwd_is_destination(
         cwd_before = Path.cwd()
         run_update(dst, defaults=True, overwrite=True)
         assert Path.cwd() == cwd_before
-    assert (dst / "fileglob.txt").read_text() == "['a.txt', 'version.txt']"
+    # NOTE: Deliberately assert only files rendered by the template, not files
+    # added by the user in the destination. Whether the latter are visible to
+    # filesystem filters during an update depends on whether the update
+    # algorithm renders into the user-specified destination, which is an
+    # implementation detail that may change.
+    # https://github.com/copier-org/copier/pull/2754#discussion_r3529453935
+    assert (dst / "fileglob.txt").read_text() == "['version.txt']"
 
 
 @pytest.mark.parametrize("subproject_path", [Path(), Path("subproject")])