tests/testlib/push-checkheads-util.sh

rust: fix unsound `OwningDirstateMap`

As per the previous patch, `OwningDirstateMap` is unsound. Self-referential
structs are difficult to implement correctly in Rust since the compiler is
free to move structs around as much as it wants to. They are also very rarely
needed in practice, so the state-of-the-art on how they should be done within
the Rust rules is still a bit new.

The crate `ouroboros` is an attempt at providing a safe way (in the Rust sense)
of declaring self-referential structs. It is getting a lot attention and was
improved very quickly when soundness issues were found in the past: rather than
relying on our own (limited) review circle, we might as well use the de-facto
common crate to fix this problem. This will give us a much better chance of
finding issues should any new ones be discovered as well as the benefit of
fewer `unsafe` APIs of our own.

I was starting to think about how I would present a safe API to the old struct
but soon realized that the callback-based approach was already done in
`ouroboros`, along with a lot more care towards refusing incorrect structs.

In short: we don't return a mutable reference to the `DirstateMap` anymore, we
expect users of its API to pass a `FnOnce` that takes the map as an argument.
This allows our `OwningDirstateMap` to control the input and output lifetimes
of the code that modifies it to prevent such issues.

Changing to `ouroboros` meant changing every API with it, but it is relatively
low churn in the end. It correctly identified the example buggy modification of
`copy_map_insert` outlined in the previous patch as violating the borrow rules.

Differential Revision: https://phab.mercurial-scm.org/D12429

# setup config and various utility to test new heads checks on push

cat >> $HGRCPATH <<EOF
[command-templates]
# simpler log output
log ="{node|short} ({phase}): {desc}\n"

[phases]
# non publishing server
publish=False

[extensions]
# we need to strip some changeset for some test cases
strip=

[experimental]
# enable evolution
evolution=all

[alias]
# fix date used to create obsolete markers.
debugobsolete=debugobsolete -d '0 0'
EOF

mkcommit() {
   echo "$1" > "$1"
   hg add "$1"
   hg ci -m "$1"
}

getid() {
   hg log --hidden --template '{node}\n' --rev "$1"
}

setuprepos() {
    echo creating basic server and client repo
    hg init server
    cd server
    mkcommit root
    hg phase --public .
    mkcommit A0
    cd ..
    hg clone server client

    if [ "$1" = "single-head" ]; then
        echo >> "server/.hg/hgrc" "[experimental]"
        echo >> "server/.hg/hgrc" "# enforce a single name per branch"
        echo >> "server/.hg/hgrc" "single-head-per-branch = yes"
    fi
}