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

script/install

@@ -174,6 +174,19 @@ $own"
     fi
 }
 
+# stop_on_own_script <fetched> <local>: stop if the local script has a
+# line the canonical file has never held, unless it is blank or a
+# comment: the repo wrote that script itself, and replacing it would
+# delete it. A repo without the script is fine.
+stop_on_own_script() {
+    [ -e "$2" ] || return 0
+    own="$(local_only_lines "$1" "$2")"
+    if [ -n "$own" ]; then
+        die "$2 has lines the canonical file has never held; rename the repo's own script to keep it:
+$own"
+    fi
+}
+
 main() {
     [ "$#" -le 1 ] || die "usage: script/install [path of a repo clone]"
 
@@ -221,6 +234,9 @@ main() {
             .golangci.yml)
                 keep_deny_entries "$tmp/$path" "$path"
                 ;;
+            script/*)
+                stop_on_own_script "$tmp/$path" "$path"
+                ;;
         esac
     done