"Cannot export conflicted branch" when updating a merged commit

[tp-woven]

Description

When jj git fetching a repo where one of my branches was just merged, I get a "Cannot export conflicted branch" error on the branch, and it shows up as conflicted despite no changes to the branch locally or remotely.

(I've seen this a few times recently, but only now got around to "documenting" it.)

Steps to Reproduce the Problem

0. (Not sure if this is related, but) The repo is backed by a .git repo.
1. Make some changes locally and upload them to GH (in my case, used jj git push --change=@).
2. (Not sure if this is related, but) jj close, but make no changes to the repo.
3. On GH, merge the PR from the pushed branch (using merge, not rebase/squash), and delete the remote branch.
4. Run jj git fetch.

Expected Behavior

main branch should get updated from the remote, and the feature branch should get deleted.

Actual Behavior

main does get updated as expected, but the feature branch is not deleted, and instead jj reports it as being conflicted.

Including the relevant jj op log, in case I need to get more information to debug this:

❯ jj op log
@ 7367ac2fa538 XXXXX 2022-08-08 12:26:08.772 +09:00 - 2022-08-08 12:26:10.042 +09:00
| fetch from git remote origin
| args: jj git fetch
o 971afe4fe51c XXXXX 2022-08-08 12:26:07.595 +09:00 - 2022-08-08 12:26:07.646 +09:00
| import git refs
| args: jj git fetch
o 9f6e121b4fa4 XXXXX 2022-08-08 12:20:17.962 +09:00 - 2022-08-08 12:20:17.992 +09:00
| close commit 14e796c3b31b2cc643ecc9e54b8236dff21db6c5
| args: jj close
o 41c1bb7f7e43 XXXXX 2022-08-08 12:20:17.858 +09:00 - 2022-08-08 12:20:17.903 +09:00
| import git refs
| args: jj close
o 906e73e750a0 XXXXX 2022-08-08 12:20:04.457 +09:00 - 2022-08-08 12:20:07.076 +09:00
| push change e7fb6c4e410147d9a0db3b4d92b028d8 to git remote origin
| args: jj uc

Edit: Just happened again without the jj close shenanigans.

[martinvonz]

As I mentioned on Discord, I think this bug typically happens like this:

1. Move branch foo from A to B
2. Export to git. This updates foo to B in Git, but doesn't update our record of foo in the Git repo
3. Move foo to C in Git
4. Import from Git. Since we didn't update our record before, we think the branch was moved from A to C

I think what you reported here is some version of that sequence, where the branch was deleted instead of moved.

I think we want to fix two things:

1. Make sure we update our record of the Git ref when we update it in Git
2. If there are branches with conflicts, skip those instead of erroring out.