uncommit: simplify condition for keeping commit
We used to keep the commit if it would have become empty and --keep
was not passed. Since we just changed it so we also keep it if any
patterns were passed on the command line, the only remaining case
where we prune the commit is when no arguments were passed and --keep
was not passed either, we can simplify the "not files and not
allowempty" to just "not allowempty". Let's also rename "allowempty"
to "keepcommit" since it's no longer about allowing an empty commit.
Differential Revision: https://phab.mercurial-scm.org/D2891
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