[PATCH b4] b4: Ignore Signed-off-by trailers in follow-up replies

When someone replies to a patch thread, sometimes they include a
Signed-off-by trailer (for example, when providing inline diffs or
snippets in a reply). Currently, b4 am/shazam collect these tags
from replies and append them to the main patch's trailers.

I am not aware of any use case when collecting the Signed-off-by
is needed, so drop it in this case.

Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Mostafa Saleh <[email protected]>

---
I hit this when using:
b4 shazam -v1 https://lore.kernel.org/all/[email protected]/

It collected the tag from Marc's patch in my patch as in:
https://lore.kernel.org/all/[email protected]/
Which is wrong.
---
 src/b4/__init__.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/b4/__init__.py b/src/b4/__init__.py
index a52b001..fd33111 100644
--- a/src/b4/__init__.py
+++ b/src/b4/__init__.py
@@ -1857,6 +1857,7 @@ class LoreMessage:
                     'base-commit',
                     'change-id',
                     'message-id',
+                    'signed-off-by',
                 }
                 if trailer.lname not in badtrailers:
                     trailer.lmsg = self
-- 
2.55.0.654.g21b8a5bc05-goog