Fix check for dirty tree in release.sh.
Fix check for dirty tree in release.sh.

BUG=skia:

Review-Url: https://codereview.chromium.org/1962973002

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot