annex merge mustn't merge into non-git-annex branc

Please describe the problem.

Somehow I ended up with the case that git remote (over ssh) had no "main" branch pushed, only "git-annex" branch which was not yet merged into local git-annex branch. More details at https://github.com/datalad/datalad/issues/7608.

git-annex info:

git-annex info shows datalad-public remote and all looks ok

(git)smaug:/mnt/btrfs/datasets/datalad/crawl/openneuro/ds002144[main]git
$> git annex info
trusted repositories: 0
semitrusted repositories: 7
        00000000-0000-0000-0000-000000000001 -- web
        00000000-0000-0000-0000-000000000002 -- bittorrent
        2606f878-85c6-459a-8402-5f4b98720bbd -- root@openneuro-prod-dataset-worker-14:/datalad/ds002144
        58a4efbe-8fb4-4cb3-8be3-b982a4673947 -- s3-PRIVATE
        72279570-1519-43aa-aea8-6f9c3a6f72f4 -- yoh@smaug:/mnt/btrfs/datasets/datalad/crawl/openneuro/ds002144 [here]
        ebcbc36d-4230-46b8-8654-87876ed1af0f -- yoh@falkor:/srv/datasets.datalad.org/www/openneuro/ds002144 [datalad-public]
        fb99edfc-26aa-40ac-b770-996e91421e88 -- [s3-PUBLIC]
untrusted repositories: 0
transfers in progress: none
available local disk space: 23.15 terabytes (+100 megabytes reserved)
local annex keys: 0
local annex size: 0 bytes
annexed files in working tree: 288
size of annexed files in working tree: 84.49 gigabytes
combined annex size of all repositories: 257.38 gigabytes (+ 90 unknown size)
annex sizes of repositories:
        88.35 GB: 2606f878-85c6-459a-8402-5f4b98720bbd -- root@openneuro-prod-dataset-worker-14:/datalad/ds002144
        85.93 GB: fb99edfc-26aa-40ac-b770-996e91421e88 -- [s3-PUBLIC]
         83.1 GB: 58a4efbe-8fb4-4cb3-8be3-b982a4673947 -- s3-PRIVATE
backend usage:
        MD5E: 288
bloom filter size: 32 mebibytes (0% full)

But the datalad-public/git-annex branch is not yet merged into local git-annex branch, and git annex merge even though says "ok", in reality does nothing - it remains not merged.

(git)smaug:/mnt/btrfs/datasets/datalad/crawl/openneuro/ds002144[main]git
$> git annex merge --debug
merge git-annex [2024-05-31 09:46:03.819131714] (Utility.Process) process [2601180] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","show-ref","git-annex"]
[2024-05-31 09:46:03.821638775] (Utility.Process) process [2601180] done ExitSuccess
[2024-05-31 09:46:03.822255445] (Utility.Process) process [2601181] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","show-ref","--hash","refs/heads/git-annex"]
[2024-05-31 09:46:03.825268573] (Utility.Process) process [2601181] done ExitSuccess
ok
[2024-05-31 09:46:03.826008662] (Utility.Process) process [2601184] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","symbolic-ref","-q","HEAD"]
[2024-05-31 09:46:03.829119829] (Utility.Process) process [2601184] done ExitSuccess
[2024-05-31 09:46:03.829536231] (Utility.Process) process [2601187] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","show-ref","refs/heads/main"]
[2024-05-31 09:46:03.833612799] (Utility.Process) process [2601187] done ExitSuccess
[2024-05-31 09:46:03.834740598] (Utility.Process) process [2601188] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","cat-file","--batch"]
[2024-05-31 09:46:03.842123731] (Utility.Process) process [2601192] call: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","show-ref","--verify","-q","refs/heads/synced/main"]
[2024-05-31 09:46:03.846432269] (Utility.Process) process [2601192] done ExitFailure 1
[2024-05-31 09:46:03.846903191] (Utility.Process) process [2601188] done ExitSuccess
(dev3) 2 12816.....................................:Fri 31 May 2024 09:46:03 AM EDT:.

$> echo $?
0

$> git br -a
  git-annex
* main
  remotes/datalad-public/git-annex
  remotes/datalad-public/main
  remotes/origin/HEAD -> origin/main
  remotes/origin/git-annex
  remotes/origin/main
  remotes/origin/master

so I thought to force the annex merge manually expecting that git-annex would do its merge into git-annex branch, but it does it into current main:

(git)smaug:/mnt/btrfs/datasets/datalad/crawl/openneuro/ds002144[main]git
$> git br -a
  git-annex
* main
  remotes/datalad-public/git-annex
  remotes/datalad-public/main
  remotes/origin/HEAD -> origin/main
  remotes/origin/git-annex
  remotes/origin/main
  remotes/origin/master

$> git branch
  git-annex
* main

$> git annex merge --allow-unrelated-histories datalad-public/git-annex
merge datalad-public/git-annex
Merge made by the 'ort' strategy.
 uuid.log | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 uuid.log
ok

$> git show
commit ce8ade971ead660c4dccc5cc1214a894fbfd65a2 (HEAD -> main)
Merge: 99006dd a7cd458
Author: Yaroslav Halchenko <debian@onerussian.com>
Date:   Fri May 31 09:47:12 2024 -0400

    Merge remote-tracking branch 'datalad-public/git-annex'

    * datalad-public/git-annex:
      update
      branch created

FWIW here is again with --debug

(git)smaug:/mnt/btrfs/datasets/datalad/crawl/openneuro/ds002144[main]git
$> git reset --hard origin/main
HEAD is now at 99006dd [DATALAD] added content
(dev3) 2 12822.....................................:Fri 31 May 2024 09:47:57 AM EDT:.
(git)smaug:/mnt/btrfs/datasets/datalad/crawl/openneuro/ds002144[main]git
$> git annex merge --debug --allow-unrelated-histories datalad-public/git-annex
merge datalad-public/git-annex [2024-05-31 09:48:03.27116017] (Utility.Process) process [2606124] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","symbolic-ref","-q","HEAD"]
[2024-05-31 09:48:03.275932771] (Utility.Process) process [2606124] done ExitSuccess
[2024-05-31 09:48:03.276628862] (Utility.Process) process [2606125] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","show-ref","refs/heads/main"]
[2024-05-31 09:48:03.281249999] (Utility.Process) process [2606125] done ExitSuccess

[2024-05-31 09:48:03.282011289] (Utility.Process) process [2606126] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","show-ref","--hash","refs/heads/main"]
[2024-05-31 09:48:03.284998952] (Utility.Process) process [2606126] done ExitSuccess
[2024-05-31 09:48:03.286186327] (Utility.Process) process [2606127] read: git ["--version"]
[2024-05-31 09:48:03.28861691] (Utility.Process) process [2606127] done ExitSuccess
[2024-05-31 09:48:03.289258819] (Utility.Process) process [2606128] call: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","annex.debug=true","merge","--no-edit","datalad-public/git-annex","--allow-unrelated-histories"]
Merge made by the 'ort' strategy.
 uuid.log | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 uuid.log
[2024-05-31 09:48:03.398901654] (Utility.Process) process [2606147] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","-c","filter.annex.smudge=","-c","filter.annex.clean=","-c","filter.annex.process=","write-tree"]
[2024-05-31 09:48:03.40183409] (Utility.Process) process [2606147] done ExitSuccess
[2024-05-31 09:48:03.402786193] (Utility.Process) process [2606148] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","--hash","refs/annex/last-index"]
[2024-05-31 09:48:03.406085518] (Utility.Process) process [2606148] done ExitSuccess
[2024-05-31 09:48:03.424223834] (Utility.Process) process [2606128] done ExitSuccess
ok

What version of git-annex are you using? On what operating system?

10.20240430+git26-g5f61667f27-1~ndall+1