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).
$ cat >> $HGRCPATH <<EOF
> [ui]
> logtemplate={rev}:{node|short}[{bookmarks}] ({obsolete}/{phase}) {desc|firstline}\n
> [extensions]
> EOF
$ echo "evolve=$(echo $(dirname $TESTDIR))/hgext3rd/evolve/" >> $HGRCPATH
$ mkcommit() {
> echo "$1" > "$1"
> hg add "$1"
> hg ci -m "add $1"
> }
$ hg init repo
$ cd repo
$ mkcommit a
$ mkcommit b
test disabling commands
$ cat >> .hg/hgrc <<EOF
> [experimental]
> evolution=createmarkers
> allowunstable
> exchange
> EOF
$ hg prune
hg: unknown command 'prune'
(use 'hg help' for a list of commands)
[255]