script/install: stop on a repo's own script of the same name

script/install replaced any tracked script/install or
script/update-policy in the target, including a repo's own unrelated
script. It now stops when such a file has a line the canonical file
has never held. An older shared copy is still replaced.

The padded file in the self-replacement test is now an earlier
canonical version, so the new stop does not catch it.

Model: opus-5-5

test/update-policy.sh

@@ -415,7 +415,8 @@ test_target_lacking_install_gets_both_scripts() {
 
 # The shell reads a script in blocks, so the padding makes it read the
 # last line only after main has returned. It finds that line only if
-# script/install left the old file in place under a new name.
+# script/install left the old file in place under a new name. The old
+# file is an earlier canonical version, so script/install replaces it.
 test_update_policy_replaces_itself() {
     start_scenario "script/update-policy replaces its own file and still exits 0"
     {
@@ -427,6 +428,10 @@ test_update_policy_replaces_itself() {
         done
         echo "echo 'the old file ran to its end'"
     } > "$dir/merged"
+    cp "$dir/merged" "$dir/canonical/script/update-policy"
+    commit_canonical
+    cp "$UPDATE_POLICY" "$dir/canonical/script/update-policy"
+    commit_canonical
     mv "$dir/merged" "$dir/repo/script/update-policy"
     chmod 755 "$dir/repo/script/update-policy"
     commit_and_push_fixture
@@ -575,6 +580,17 @@ EOF
     expect_no_commit
 }
 
+test_own_script_stops() {
+    start_scenario "a tracked script/install with a line the canonical file has never held stops"
+    echo "echo our own installer" >> "$dir/repo/script/install"
+    commit_and_push_fixture .prettierrc
+    run_update_policy
+    expect_failure
+    expect_output "script/install has lines"
+    expect_output "echo our own installer"
+    expect_nothing_written
+}
+
 test_gitignore_lacks_canonical_last_line() {
     start_scenario ".gitignore lacking the canonical last line stops"
     printf '*.log\n/myapp\n' > "$dir/repo/.gitignore"
@@ -688,6 +704,12 @@ test_former_canonical_gitignore_line_replaced() {
     expect_former_canonical_line_replaced .gitignore "*.old"
 }
 
+test_former_canonical_script_line_replaced() {
+    start_scenario "a script/install line an earlier canonical commit held is replaced"
+    expect_former_canonical_line_replaced script/install "echo an older install"
+    expect_same_as_canonical script/install
+}
+
 # A line another file's history removed does not count as held by this
 # one.
 test_line_removed_from_another_file_still_stops() {
@@ -722,6 +744,7 @@ main() {
     test_own_entry_above_closing_comment
     test_uncommitted_change_stops
     test_untracked_script_stops
+    test_own_script_stops
     test_gitignore_lacks_canonical_last_line
     test_fetched_golangci_without_deny_list
     test_own_golangci_setting_stops
@@ -730,6 +753,7 @@ main() {
     test_update_policy_takes_no_argument
     test_former_canonical_golangci_line_replaced
     test_former_canonical_gitignore_line_replaced
+    test_former_canonical_script_line_replaced
     test_line_removed_from_another_file_still_stops
     echo "update-policy tests passed"
 }