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).
$ . ${TESTDIR}/testlib/pythonpath.sh
$ cat >> $HGRCPATH <<EOF
> [defaults]
> amend=-d "0 0"
> [web]
> push_ssl = false
> allow_push = *
> [phases]
> publish = False
> [experimental]
> bundle2-exp=False # < Mercurial-4.0
> [devel]
> legacy.exchange=bundle1
> [extensions]
> EOF
$ mkcommit() {
> echo "$1" > "$1"
> hg add "$1"
> hg ci -m "add $1"
> }
$ hg init server
Try the multiple ways to setup the extension
$ hg -R server log --config 'extensions.evolve.serveronly='
$ hg -R server log --config "extensions.evolve.serveronly=${SRCDIR}/hgext3rd/evolve/serveronly.py"
$ PYTHONPATH=$HGTEST_ORIG_PYTHONPATH hg -R server log --config "extensions.evolve.serveronly=${SRCDIR}/hgext3rd/evolve/serveronly.py"
setup repo
$ echo "[extensions]" >> ./server/.hg/hgrc
$ echo "evolve.serveronly=" >> ./server/.hg/hgrc
$ hg serve -R server -n test -p $HGPORT -d --pid-file=hg.pid -A access.log -E errors.log --traceback
$ cat hg.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/ client
no changes found
updating to branch default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cat ./errors.log
$ echo "[extensions]" >> ./client/.hg/hgrc
$ echo "evolve=" >> ./client/.hg/hgrc
$ cp -r client other
Smoke testing
===============
$ cd client
$ mkcommit 0
$ mkcommit a
$ hg push
pushing to http://localhost:$HGPORT/
searching for changes
abort: remote error:
incompatible Mercurial client; bundle2 required
(see https://www.mercurial-scm.org/wiki/IncompatibleClient)
[255]
$ cat ../errors.log