Use absolute path for art/tools/cpplint.py --root

art/tools/cpplint.py still fails when run with --root=. if the
.git directory is missing.  Replace --root=. with --root=$(pwd)
to give it an absolute path.

Bug: 34787652
Test: mv art/.git art/.git.bak; m -j cpplint-art-phony; mv art/.git.bak art/.git
Change-Id: I56c2acf54090e53a02d52fb12ffd2e5a862ede4f