uncommit: add options to update to the current user or current date These are also from the evolve extension's version of uncommit. I tried adding validation that both forms of user or date can't be specified at the same time, but that fails because these show up in `opts` with a None value whether or not the option was given on the command line. Presumably that means the conditional in `resolvecommitoptions` could be simplified. But this is how both evolve and MQ handle it. Differential Revision: https://phab.mercurial-scm.org/D6828