Optimize CheckForCommitObjects by batching git ls-tree operations

Significantly reduces the execution time of presubmit checks by
optimizing CheckForCommitObjects.

For CLs with fewer than 1000 affected files, run `git ls-tree` only on
specific files instead of scanning the full tree. This yields a ~70x
speedup (~0.97s -> ~0.01s) for typical CLs.

Change-Id: Ia8b89dbb14a5129ba79944282deba52a3558bdf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/7234371
Commit-Queue: Josiah Kiehl <kiehl@google.com>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

tests/presubmit_canned_checks_test.py

@@ -757,9 +757,6 @@ class CheckAyeAyeTest(unittest.TestCase):
         self.assertEqual(results[0].type, 'error')
         self.assertIn("Failed to run.", results[0].message)
 
-if __name__ == '__main__':
-    unittest.main()
-
 
 class CheckForCommitObjectsTest(unittest.TestCase):
 
@@ -772,6 +769,7 @@ class CheckForCommitObjectsTest(unittest.TestCase):
         self.patcher = mock.patch('presubmit_canned_checks._ParseDeps')
         self.mock_parse_deps = self.patcher.start()
         self.mock_parse_deps.return_value = {'git_dependencies': 'DEPS'}
+        self.input_api.change.RepositoryRoot = lambda: ''
 
     def tearDown(self):
         self.patcher.stop()
@@ -865,3 +863,53 @@ class CheckForCommitObjectsTest(unittest.TestCase):
         results = presubmit_canned_checks.CheckForCommitObjects(
             self.input_api, self.output_api)
         self.assertEqual(0, len(results))
+
+    def testRunFromSubdir_SmallFiles_NoSubmodules(self):
+        self.input_api.presubmit_local_path = os.path.join(ROOT_DIR, 'subdir')
+        self.input_api.change.RepositoryRoot = lambda: ROOT_DIR
+        self.input_api.files = [MockAffectedFile('foo.txt', 'content')]
+        self.input_api.subprocess.check_output.return_value = b''
+
+        results = presubmit_canned_checks.CheckForCommitObjects(
+            self.input_api, self.output_api)
+        self.assertEqual(0, len(results))
+
+    def testRunFromSubdir_SmallFiles_WithSubmodules(self):
+        self.input_api.presubmit_local_path = os.path.join(ROOT_DIR, 'subdir')
+        self.input_api.change.RepositoryRoot = lambda: ROOT_DIR
+        self.input_api.files = [MockAffectedFile('foo.txt', 'content')]
+        self.input_api.subprocess.check_output.return_value = b'160000 commit 1234\tsubmodule\0'
+
+        results = presubmit_canned_checks.CheckForCommitObjects(
+            self.input_api, self.output_api)
+        self.assertEqual(1, len(results))
+        self.assertIn('submodule', results[0].items)
+
+    def testRunFromSubdir_LargeFiles_NoSubmodules(self):
+        self.input_api.presubmit_local_path = os.path.join(ROOT_DIR, 'subdir')
+        self.input_api.change.RepositoryRoot = lambda: ROOT_DIR
+        self.input_api.files = [
+            MockAffectedFile(f'f{i}', '') for i in range(1001)
+        ]
+        self.input_api.subprocess.check_output.return_value = b''
+
+        results = presubmit_canned_checks.CheckForCommitObjects(
+            self.input_api, self.output_api)
+        self.assertEqual(0, len(results))
+
+    def testRunFromSubdir_LargeFiles_WithSubmodules(self):
+        self.input_api.presubmit_local_path = os.path.join(ROOT_DIR, 'subdir')
+        self.input_api.change.RepositoryRoot = lambda: ROOT_DIR
+        self.input_api.files = [
+            MockAffectedFile(f'f{i}', '') for i in range(1001)
+        ]
+        self.input_api.subprocess.check_output.return_value = b'160000 commit 1234\tsubmodule\0'
+
+        results = presubmit_canned_checks.CheckForCommitObjects(
+            self.input_api, self.output_api)
+        self.assertEqual(1, len(results))
+        self.assertIn('submodule', results[0].items)
+
+
+if __name__ == '__main__':
+    unittest.main()