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).
Test that share-clones use the cache in the original repository.
$ . $TESTDIR/testlib/common.sh
$ hg init share-base
$ cd share-base/
$ cat >> .hg/hgrc <<EOF
> [extensions]
> evolve=
> share=
> EOF
$ hg debugbuilddag .+3:branchpoint+4*branchpoint+2
$ cd ..
$ hg --config extensions.share= share -U share-base share-client
$ cd share-client
$ hg debugupdatecache
$ test -d .hg/cache
[1]
$ test -d ../share-base/.hg/cache