file_list can be none if gclient runs with --nohooks.
Fix apply_patch_ref so that it doesn't try to update file_list when it is
None.

TBR=agable@chromium.org

Bug: 643346
Change-Id: If00547da004415edfe68196a44cbda753b4db017
Reviewed-on: https://chromium-review.googlesource.com/989279
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>