uncommit: add config option to keep commit by default We have a Google-internal extension that keeps track of "review units" (like Phabricator reviews, or Gerrit's Change-Id). This information is stored outside of the commit. It is updated with rewrites. Every now and then we get reports from users who are confused because `hg uncommit` lost track of their review. Keeping the empty commit by default would reduce this confusion. It may also cause confusion about the empty commit. This patch adds a config option that lets us easily test both behaviors on our users. Differential Revision: https://phab.mercurial-scm.org/D5970