This fixes the case where a new workdir is created from a repository
which was previously created with gclient-new-workdir.py and that
repository is later deleted, rendering the symbolic links created by
gclient-new-workdir.py invalid.

It also fixes support_cow() returning 'True' in that case because 'cp
--reflink' does not fail when copying a symlink.


Bug: 721585
Change-Id: I84f4d80b8698af0da346be559d46a328ec0d0e9b
Reviewed-on: https://chromium-review.googlesource.com/842402Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>