tests: demonstrate content divergence causing "relocate node on top of itself"

cmdstate: introduce a "saver" contextmanager and use it in place of save() Previously, the state was only saved in some paths out of these functions. This can be problematic, if the user ctrl-c's (or `kill -9`'s) the process, or we exit out of `relocate` for anything besides the "expected" reason, we won't record that we were in the middle of an evolve. One of our users has discovered that this leaves hg in a weird state; the user did something like this: ``` $ hg evolve <something goes wrong with the merge tool, hits ctrl-c> <deals with the merge conflicts> $ hg evolve --continue abort: no interrupted evolve to continue $ hg evolve abort: uncommitted changes # Note: commands.status.verbose=True is set. $ hg status M foo # The repository is in an unfinished *update* state. # No unresolved merge conflicts # To continue: hg update ``` The user did an `hg update`, but it didn't actually do anything besides take it out of the unfinished update state (the files were still dirty in the working directory).