`b4 prep --auto-to-cc` doesn't recognise trailers anymore

[0xIO32]

Since bf7beca, b4 prep --auto-to-cc doesn't recognise the existing To: and Cc: trailers anymore. Instead, it just adds them at the bottom of the cover.

The issue only occurs in the 2. patch version or higher, as b4 now has added

---
Changes in v2:
- changes
- Link to v1: <some link>

to the cover letter.

For instance:

Example subject

Example body

To: Example <test@example.com>
Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
Changes in v2:
- some change
- Link to v1: <some link>

results in

Example subject

Example body

To: Example <test@example.com>
Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
Changes in v2:
- some change
- Link to v1: <some link>

To: Example <test@example.com>

after the use of b4 prep --auto-to-cc

My current cover template:

${cover}

---
${shortlog}

${diffstat}
---
base-commit: ${base_commit}
change-id: ${change_id}
${prerequisites}