Fix migration planner to fully understand squashed migrations. And test.

tests/migrations/test_migrations_squashed/0001_squashed_0002.py

@@ -0,0 +1,32 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    replaces = [
+        ("migrations", "0001_initial"),
+        ("migrations", "0002_second"),
+    ]
+
+    operations = [
+
+        migrations.CreateModel(
+            "Author",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("name", models.CharField(max_length=255)),
+                ("slug", models.SlugField(null=True)),
+                ("age", models.IntegerField(default=0)),
+                ("rating", models.IntegerField(default=0)),
+            ],
+        ),
+
+        migrations.CreateModel(
+            "Book",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("author", models.ForeignKey("migrations.Author", null=True)),
+            ],
+        ),
+
+    ]