unshelve: add abort on using continue and interactive together `unshelve --continue --interactive` will not work as expected by the user as the mode of in-progress unshelve is preserved and cannot be overwritten. This patch makes `unshelve` to throw an error on using both `--continue` and `--interactive` together with `unshelve`. Differential Revision: https://phab.mercurial-scm.org/D6703