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