Fix gclient-new-workdir when source repo is shallow clone

When source repo is a 'shallow clone', `.git/shallow` is not linked to
the new workdir. Which makes running git command fails in new workdir.

Change-Id: I81d38b7b2cccc482711dcc1dd02ea06fbd01a56d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2211635
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Richard He <linyhe@microsoft.com>
diff --git a/git_common.py b/git_common.py
index 182fc0c..9550085 100644
--- a/git_common.py
+++ b/git_common.py
@@ -1083,6 +1083,7 @@
     'refs',
     'remotes',
     'rr-cache',
+    'shallow',
   ]
   make_workdir_common(repository, new_workdir, GIT_DIRECTORY_WHITELIST,
                       ['HEAD'])