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
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL