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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.