new_variant: warn user about pending changes

If there are uncommitted changes or if the local branch is ahead of
upstream, warn the user and have them fix it (or override it) before
continuing.

BUG=b:176124406
TEST=follow instructions in platform/dev/contrib/variant/testdata/README.md

Change-Id: Ic2f50029b7c43718d92e55a4dd627a93df234cc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2600106
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Justin TerAvest <teravest@chromium.org>
8 files changed