generate_secrets: Fix handling of missing trailing newlines.

When we added support for automatically adding new secrets in
generate_secrets.py, we failed to account for the possibility that a
human editor might have let the secrets file without a trailing
newline.

We address this by adding a leading newline before our new secret.

Fixes #5209.

scripts/setup/generate_secrets.py

@@ -108,7 +108,9 @@ def generate_secrets(development=False):
         return
 
     out = open(OUTPUT_SETTINGS_FILENAME, 'a')
-    out.write(force_str("".join(lines)))
+    # Write a newline at the start, in case there was no newline at
+    # the end of the file due to human editing.
+    out.write("\n" + force_str("".join(lines)))
     out.close()
 
     print("Generated new secrets in %s." % (OUTPUT_SETTINGS_FILENAME,))