fix: count files shared by systems apart

The systems step and --list-systems gave 3do 1704 files, 349 MB, of
which 1689 are MAME hash and plugin files six systems carry. Each line
now shows the system's own files, and the shared ones beside them.

tests/test_install_selection.py

@@ -426,6 +426,42 @@ class ACoreWithNoFileOfItsOwn(unittest.TestCase):
         self.assertIn("always kept", line)
 
 
+class FilesSharedBySystemsAreCountedApart(unittest.TestCase):
+    """`3do 1704 files, 349.0 MB`: 1689 of them are MAME's hash and plugin
+    files, which six systems carry. Picking 3DO, one expects its BIOS."""
+
+    FILES = [
+        {"dest": "panafz10.bin", "size": 4, "cores": None, "systems": ["3do"]},
+        {"dest": "mame/hash/a.xml", "size": 100, "cores": None,
+         "systems": ["3do", "philips-cdi"]},
+        {"dest": "mame/hash/b.xml", "size": 50, "cores": None,
+         "systems": ["3do", "philips-cdi"]},
+        {"dest": "gba_bios.bin", "size": 8, "cores": None, "systems": ["nintendo-gba"]},
+    ]
+
+    def _listing(self) -> str:
+        out = io.StringIO()
+        with contextlib.redirect_stdout(out):
+            install._print_choices("retroarch", self.FILES, [], systems=True)
+        return out.getvalue()
+
+    def test_the_listing(self):
+        lines = {line.split()[0]: line for line in self._listing().splitlines()}
+        self.assertIn("1 file, 4 B, and 2 files, 150 B shared with other systems",
+                      lines["3do"])
+        self.assertIn("2 files, 150 B, all shared with other systems", lines["philips-cdi"])
+        self.assertTrue(lines["nintendo-gba"].endswith("1 file, 8 B"))
+
+    def test_the_systems_step(self):
+        answers = iter(["c", ""])
+        out = io.StringIO()
+        with mock.patch("builtins.input", lambda _prompt="": next(answers)), \
+                contextlib.redirect_stdout(out):
+            install._prompt_custom_selection("retroarch", self.FILES, [])
+        self.assertIn("1 file, 4 B, and 2 files, 150 B shared with other systems",
+                      out.getvalue())
+
+
 class TheBoundaryChecksTheNewFields(unittest.TestCase):
     def _manifest(self, **extra):
         entry = {"dest": "a.bin", "sha1": "a" * 40, "size": 1,