fix: keep a RetroPie list's comma before its and

quasi88 writes its list "N88KNJ2.ROM, and N88SUB.ROM". Extended with
N88EXT0.ROM, which QUASI88kai reads, it came back as "N88SUB.ROM and
N88EXT0.ROM": every list was rewritten in one idiom. A list that puts a
comma before its final and keeps it.

scripts/exporter/retropie_exporter.py

@@ -250,11 +250,13 @@ class Exporter(BaseExporter):
         return core in required_by
 
     @staticmethod
-    def _join(names: list[str]) -> str:
-        """RetroPie's own idiom: a, b and c."""
+    def _join(names: list[str], serial: bool = False) -> str:
+        """RetroPie's own idiom: a, b and c, or a, b, and c where the
+        sentence already writes the comma before its final and."""
         if len(names) == 1:
             return names[0]
-        return ", ".join(names[:-1]) + " and " + names[-1]
+        last = ", and " if serial and len(names) > 2 else " and "
+        return ", ".join(names[:-1]) + last + names[-1]
 
     @classmethod
     def _extended(
@@ -269,7 +271,8 @@ class Exporter(BaseExporter):
         name: appending ", c and d" after "a and b" read "a and b, c and d".
         """
         start, end, written = enumeration
-        return help_text[:start] + cls._join([*written, *missing]) + help_text[end:]
+        serial = ", and " in help_text[start:end]
+        return help_text[:start] + cls._join([*written, *missing], serial) + help_text[end:]
 
     def modules(self, originals: dict[str, str]) -> dict[str, tuple[str, str]]:
         """Module id and help string of every script that mentions BIOS."""

tests/test_export_counts.py

@@ -700,6 +700,16 @@ class RetroPieProposals(unittest.TestCase):
         self.assertIn("gexpress.pce", text)
         self.assertNotIn("pac-n1.bin", text)
 
+    def test_an_extended_list_keeps_its_own_final_comma(self):
+        """quasi88 writes "N88KNJ2.ROM, and N88SUB.ROM": the list came back
+        without its comma before the final and."""
+        help_text = "Copy bios files A.ROM, B.ROM, and C.ROM to $biosdir/pc88"
+        enumeration = RetroPie._enumeration(help_text)
+        self.assertEqual(
+            RetroPie._extended(help_text, enumeration, ["D.ROM"]),
+            "Copy bios files A.ROM, B.ROM, C.ROM, and D.ROM to $biosdir/pc88",
+        )
+
     def test_a_list_of_alternatives_is_not_extended(self):
         alternatives = "Copy the required BIOS file a.rom or b.rom to $biosdir"
         enumeration = "Copy the required BIOS files a.rom and b.rom to $biosdir"