Need to run git cache exists in a directory we know for sure that exists

We don't want to run it in the DEPS directory, which is apperently what
the default behavior is.

BUG=339171

Review URL: https://codereview.chromium.org/184793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@254266 0039d316-1c4b-4281-b951-d872f2087c98

gclient_scm.py

@@ -685,7 +685,7 @@ class GitWrapper(SCMWrapper):
               ['--shallow', '--cache-dir', self.cache_dir, url],
               options, cwd=self._root_dir, retry=True)
     return self._Run(['cache', 'exists', '--cache-dir', self.cache_dir, url],
-                     options).strip()
+                     options, cwd=self._root_dir, ).strip()
 
   def _Clone(self, revision, url, options):
     """Clone a git repository from the given URL.