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