pr-pull: clean up review signoff trailer code

[jonchang]

This should avoid situations that mangle the trailers when trailers already exist but we want to add review signoff trailers. After this pull request, the commit messages look like:

Homebrew/homebrew-core@c6a0ed8

r 4.0.3

* r 4.0.3
* r: update license

Closes #62704.

Co-authored-by: chenrui <chenrui333@gmail.com>
Signed-off-by: FX Coudert <fxcoudert@gmail.com>
Signed-off-by: Jonathan Chang <me@jonathanchang.org>

Homebrew/homebrew-core@7db0bc2

conan 1.30.1

Closes #62710.

Signed-off-by: Rui Chen <rui@meetup.com>
Signed-off-by: FX Coudert <fxcoudert@gmail.com>
Signed-off-by: BrewTestBot <1589480+BrewTestBot@users.noreply.github.com>

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