uncommit: don't allow bare uncommit on dirty working directory
As per IRC discussion and suggestion by Augie, we should not allow bare `hg
uncommit` on dirty working directory.
Differential Revision: https://phab.mercurial-scm.org/D734
uncommit: rename the flag 'empty' to 'keep' which retains empty changeset
This patch renames the flag 'empty' which retains the empty changeset if all
the files are uncommitted to 'keep'.
Differential Revision: https://phab.mercurial-scm.org/D726