cinnabar crashes with core dump rather than failing gracefully if upstream hg rejects push

[riastradh]

#!/bin/sh

set -Ceux

hg init repo.hg
echo hello >repo.hg/README
hg -R repo.hg --config ui.username='a <b@c>' commit -Am 'initial tree'

git clone hg::"$(pwd)/repo.hg" repo.git
echo world >>repo.git/README
git -C repo.git -c user.name=e -c user.email=f@g commit -am 'updated tree'

# Inject an error.
cat >>repo.hg/.hg/hgrc <<EOF
[hooks]
pretxnclose = python:/nonexistent:fali
EOF

git -C repo.git push

Output:

...
+ git -C repo.git push
Bundling 1 changesets
Bundling 1 manifests
Bundling 1 revisions of 1 files
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: loading pretxnclose hook failed:
remote: transaction abort!
remote: rollback completed
remote: abort: No such file or directory: '/nonexistent'
fatal: called `Result::unwrap()` on an `Err` value: Error { kind: UnexpectedEof, message: "failed to fill whole buffer" }
Run the command again with `git -c cinnabar.check=traceback <command>` to see the full traceback.
error: git-remote-hg died of signal 6
error: failed to push some refs to 'hg::/home/riastradh/tmp/20241026/cinnatest/repo.hg'

Expected output: nothing involving unwrap on error value, no core dump, since the cause of the failed push is not (directly) a bug in Rust code but a refusal by the upstream repo.