git checkout in colocated repo may abandon old HEAD

[yuja]

Description

In colocated repo, if you run jj after git checkout (not jj checkout), old HEAD could be abandoned.
This behavior is correct since the HEAD ref is moved externally, and the old HEAD is no longer referenced by git, but it's easy to lose commits by mistake.

I sometimes do git fetch && git checkout FETCH_HEAD as sloppy way of switching branches, and notice my wip commits went away after that.

Steps to Reproduce the Problem

set -e
set -x

#rm -Rf repo
mkdir repo
cd repo

git init
jj init --git-repo .

touch a
jj ci -m 'jj a'

touch b
jj ci -m 'jj b'

jj log --no-pager

# move HEAD ref back without using jj: 'b' -> 'a'
git checkout `jj log -r 'description("jj a")' -T 'commit_id' --no-graph`

# git::import_refs() abandons old HEAD 'b'
sleep 1
jj log --no-pager

Specifications

• Platform: Linux
• Version: d5bbf65