script/install: do not stop on a missing closing comment

A .gitignore or .dockerignore lacking the canonical last line no longer
stops the script: each line the canonical file has never held goes
below the fetched content. An .editorconfig lacking it is replaced,
unless it has such a line, which stops the script.

The stub curl now writes files with no execute bit, as a real download
does, so the tests exercise the mode the script sets.

Model: opus-5-5

script/install

@@ -15,6 +15,12 @@
 #   - .gitignore, .editorconfig, .dockerignore: the lines below the
 #     canonical file's last line. An entry above that line which the
 #     canonical file has never held stops the script.
+#   - .gitignore, .dockerignore lacking that last line: each line the
+#     canonical file has never held, unless it is blank or a comment,
+#     goes below the canonical content.
+#   - .editorconfig lacking that last line: it is replaced if it has no
+#     such line. Otherwise the script stops, because a setting means
+#     nothing away from its section.
 #   - .golangci.yml: the entries the repo added to the deny list of the
 #     test-support rule. Any other line the canonical file has never
 #     held stops the script, unless it is blank or a comment.
@@ -117,12 +123,23 @@ local_only_lines() {
 # keep_lines_below_last <fetched> <local>: append to the fetched file
 # whatever follows its last line in the local file. Stop if the canonical
 # file has never held a line above that one, unless it is blank or a
-# comment: replacing the file would delete it.
+# comment: replacing the file would delete it. If the local file lacks
+# that last line, append each line the canonical file has never held,
+# unless it is blank or a comment. An .editorconfig with such a line
+# stops the script instead, because a setting means nothing away from
+# its section.
 keep_lines_below_last() {
     last="$(tail -n 1 "$1")"
     number="$(grep -n -F -x -- "$last" "$2" | head -n 1 | cut -d: -f1)"
     if [ -z "$last" ] || [ -z "$number" ]; then
-        die "$2 lacks this line; add it and put the repo's own entries below it: $last"
+        own="$(local_only_lines "$1" "$2")"
+        [ -n "$own" ] || return 0
+        if [ "$2" = .editorconfig ]; then
+            die "$2 lacks this line and has lines the canonical file has never held; add it and put those lines below it: $last
+$own"
+        fi
+        printf '%s\n' "$own" >> "$1"
+        return 0
     fi
     own="$(local_only_lines "$1" "$2" "$number")"
     if [ -n "$own" ]; then