Setting this to `dirty` is generally desirable. If the user has
explicitly set it to something else on their chromium checkout,
warn rather than silently overwriting.
Rev^2: https://crrev.com/c/5832742
Change-Id: I8392c7976af0a1f8754a630bd865830ba6698051
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5836831
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Helmut Januschka <helmut@januschka.com>