Add ListSubmodules helper for presubmits.

We need this for this presubmit otherwise we cannot remove git deps from DEPS: http://shortn/_P7Xkt33SRx
We will also be able to reuse this and replace: http://shortn/_WYBIUC9pa4

Bug: 1475770
Change-Id: I4cea689d130df77c344d82d4d8f9db02d42cd847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5209098
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

presubmit_support.py

@@ -1336,6 +1336,10 @@ class Change(object):
         """Convenience function."""
         return [af.AbsoluteLocalPath() for af in self.AffectedFiles()]
 
+    def ListSubmodules(self):
+        """Returns submodule paths for current change's repo."""
+        return scm.GIT.ListSubmodules(self.change.RepositoryRoot())
+
     def RightHandSideLines(self):
         """An iterator over all text lines in 'new' version of changed files.
 

scm.py

@@ -455,6 +455,17 @@ class GIT(object):
             return VERSIONED_SUBMODULE
         return VERSIONED_DIR
 
+    @staticmethod
+    def ListSubmodules(repo_root):
+        # type: (str) -> Collection[str]
+        """Returns the list of submodule paths for the given repo."""
+        if not os.path.exists(os.path.join(repo_root, '.gitmodules')):
+            return []
+        config_output = GIT.Capture(
+            ['git', 'config', '--file', '.gitmodules', '--get-regexp', 'path'],
+            cwd=repo_root)
+        return [line.split()[-1] for line in config_output.splitlines()]
+
     @staticmethod
     def CleanupDir(cwd, relative_dir):
         """Cleans up untracked file inside |relative_dir|."""

tests/scm_unittest.py

@@ -132,6 +132,18 @@ class GitWrapperTestCase(unittest.TestCase):
         actual_state = scm.GIT.IsVersioned('cwd', 'dir')
         self.assertEqual(actual_state, scm.VERSIONED_DIR)
 
+    @mock.patch('scm.GIT.Capture')
+    @mock.patch('os.path.exists', return_value=True)
+    def testListSubmodules(self, mockExists, mockCapture):
+        mockCapture.return_value = (
+            'submodule.submodulename.path foo/path/script'
+            '\nsubmodule.submodule2name.path foo/path/script2')
+        actual_list = scm.GIT.ListSubmodules('root')
+        self.assertEqual(actual_list, ['foo/path/script', 'foo/path/script2'])
+
+    def testListSubmodules_missing(self):
+        self.assertEqual(scm.GIT.ListSubmodules('root'), [])
+
 
 class RealGitTest(fake_repos.FakeReposTestBase):
     def setUp(self):