avoid starting unnecessary processes when file count is limited

Author: asottile

src/flake8/checker.py

@@ -256,6 +256,7 @@ def start(self) -> None:
                 exclude=self.exclude,
             )
         )
+        self.jobs = min(len(self.filenames), self.jobs)
 
     def stop(self) -> None:
         """Stop checking files."""

tests/unit/test_checker_manager.py

@@ -61,6 +61,16 @@ def test_multiprocessing_cpu_count_not_implemented():
     assert manager.jobs == 0
 
 
+def test_jobs_count_limited_to_file_count():
+    style_guide = style_guide_mock()
+    style_guide.options.jobs = JobsArgument("4")
+    style_guide.options.filenames = ["file1", "file2"]
+    manager = checker.Manager(style_guide, finder.Checkers([], [], []), [])
+    assert manager.jobs == 4
+    manager.start()
+    assert manager.jobs == 2
+
+
 def test_make_checkers():
     """Verify that we create a list of FileChecker instances."""
     style_guide = style_guide_mock()