remove one other instance of `git ls-files`

Change-Id: I44d335eb9cc93442b592758733f1912ba46fddf8
Reviewed-on: https://gerrit.instructure.com/31218
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Raphael Weiner <rweiner@pivotallabs.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>