This command archives branches whose Rietveldt status is closed by
creating new Git tags for each of the branches' heads, and then
deleting the branch. It automatically cleans up the clutter that
accumulates over time in a long-lived Git checkout.

For example, the branch "foo-bar" associated with the
closed issue 1568403002 will be archived to the tag
"git-cl-archived-1568403002-foo-bar".

BUG=616404
R=martiniss@chromium.org,tandrii@chromium.org

Review-Url: https://codereview.chromium.org/1991563005