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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state