[BUG] b4: assertion error with `b4 send`

Hi,

I got this assertion error when sending patches today:

---
Sent 11 messages
Tagging sent/20250305-unique-ref-29fcd675f9e9-v17
Recording series message-id in cover letter tracking
Created new revision v18
Updating cover letter with templated changelog entries.
Invoking git-filter-repo to update the cover letter.
New history written in 0.06 seconds...
Traceback (most recent call last):
  File "/nix/store/rnbga7cjzjihq8qayhm2fdpq18g49dwm-b4-0.15.2/bin/.b4-wrapped", line 9, in <module>
    sys.exit(cmd())
             ~~~^^
  File "/nix/store/rnbga7cjzjihq8qayhm2fdpq18g49dwm-b4-0.15.2/lib/python3.13/site-packages/b4/command.py", line 512, in cmd
    cmdargs.func(cmdargs)
    ~~~~~~~~~~~~^^^^^^^^^
  File "/nix/store/rnbga7cjzjihq8qayhm2fdpq18g49dwm-b4-0.15.2/lib/python3.13/site-packages/b4/command.py", line 95, in cmd_send
    b4.ez.cmd_send(cmdargs)
    ~~~~~~~~~~~~~~^^^^^^^^^
  File "/nix/store/rnbga7cjzjihq8qayhm2fdpq18g49dwm-b4-0.15.2/lib/python3.13/site-packages/b4/ez.py", line 2352, in cmd_send
    reroll(mybranch, tag_msg, cl_msgid)
    ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/rnbga7cjzjihq8qayhm2fdpq18g49dwm-b4-0.15.2/lib/python3.13/site-packages/b4/ez.py", line 2491, in reroll
    store_cover(new_cover, tracking)
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/rnbga7cjzjihq8qayhm2fdpq18g49dwm-b4-0.15.2/lib/python3.13/site-packages/b4/ez.py", line 749, in store_cover
    run_frf(frf)
    ~~~~~~~^^^^^
  File "/nix/store/rnbga7cjzjihq8qayhm2fdpq18g49dwm-b4-0.15.2/lib/python3.13/site-packages/b4/ez.py", line 157, in run_frf
    frf.run()
    ~~~~~~~^^
  File "/nix/store/xl7fx5x8g52g03npg1dpjzvaiijpv98z-python3.13-git-filter-repo-2.47.0/lib/python3.13/site-packages/git_filter_repo.py", line 4936, in run
    self._record_metadata(self.results_tmp_dir(), self._orig_refs)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/xl7fx5x8g52g03npg1dpjzvaiijpv98z-python3.13-git-filter-repo-2.47.0/lib/python3.13/site-packages/git_filter_repo.py", line 4767, in _record_metadata
    self._compute_metadata(metadata_dir, orig_refs)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/xl7fx5x8g52g03npg1dpjzvaiijpv98z-python3.13-git-filter-repo-2.47.0/lib/python3.13/site-packages/git_filter_repo.py", line 4712, in _compute_metadata
    assert(usoa == intermediate) # old wasn't pruned => usoa == intermediate


It looks like everything is OK and b4 set up for the next spin, cover
letter looks right.

I observed this on nixos first with b4 0.14.3 via nixpkgs 25.05, then
upgraded to 0.15.2 via nixpkgs unstable.

Best regards,
Andreas