Update migrations list file behaviour

Instead of using the file path when the git commit is not
added, use the file and the migrations list no matter if we select
all migrations or just from a certain git commit.
And unit tests :)

tests/unit/test_linter.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import unittest
+import tempfile
 
 from django.db.migrations import Migration
 
@@ -98,3 +99,42 @@ class LinterFunctionsTestCase(unittest.TestCase):
         linter = MigrationLinter(exclude_migration_tests=["NOT_NULL"], database="mysql")
         linter.lint_migration(m)
         self.assertFalse(linter.has_errors)
+
+    def test_read_migrations_unknown_file(self):
+        file_path = "unknown_file"
+        migration_list = MigrationLinter.read_migrations_list(file_path)
+        self.assertEqual([], migration_list)
+
+    def test_read_migrations_empty_file(self):
+        with tempfile.NamedTemporaryFile() as tmp:
+            migration_list = MigrationLinter.read_migrations_list(tmp.name)
+            self.assertEqual([], migration_list)
+
+    def test_read_migrations_from_file(self):
+        tmp = tempfile.NamedTemporaryFile(mode="w", delete=False)
+        tmp.write(
+            "test_project/app_add_not_null_column/migrations/0001_create_table.py\n"
+        )
+        tmp.write("unknown\n")
+        tmp.write(
+            "test_project/app_add_not_null_column/migrations/0002_add_new_not_null_field.py\n"
+        )
+        tmp.close()
+        migration_list = MigrationLinter.read_migrations_list(tmp.name)
+        self.assertEqual(
+            [
+                ("app_add_not_null_column", "0001_create_table"),
+                ("app_add_not_null_column", "0002_add_new_not_null_field"),
+            ],
+            migration_list,
+        )
+
+    def test_gather_migrations_with_list(self):
+        linter = MigrationLinter()
+        migrations = linter._gather_all_migrations(
+            migrations_list=[
+                ("app_add_not_null_column", "0001_create_table"),
+                ("app_add_not_null_column", "0002_add_new_not_null_field"),
+            ]
+        )
+        self.assertEqual(2, len(list(migrations)))