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>

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|."""