Linus Torvalds [off-list ref] writes:

So I'd really like some way to not see it.

Ted suggested a NUL character in the commit message in front of the
"hidden content". What do you think?

You do not have to resort to NUL; we could just stuff whatever you do not
need to see but needs to be left *intact* in the new header fields just
like the embedded GPG signatures are stored in signed commits.

By the time the integrator is presented the merge commit template, we
would have:

 1. The merge title (e.g. "Merge tag for-linus of git://.../rusty.git/");

 2. Payload of the signed tag (or just "annotated tag"), which is used to
    convey meaningful topic description from the lieutenant;

 3. The signature in the tag, if the tag is not just merely annotated, but
    is signed;

 4. The output from GPG verification of the above (only when 3. is
    available); and

 5. The traditional "merge summary", if merge.log is enabled.

The 10-patch series I sent earlier appends 2 and 3 with "tag:" prefix and
4 with "# " prefix in the commit log template, but it does not have to be
that way. We could arrange things so that we put only 1, 2, 4 (still with
"# " prefix because this is meant to help you verify the authenticity, not
for later third-party audit, and to be stripped away with stripspace
before the commit is made) and 5 in the commit log template, and the
original signed tag contents (only when the tag is signed, not merely
annotated) in a separate file MERGE_SIG in $GIT_DIR/ next to MERGE_MSG,
and teach "git commit" to pick it up and stuff it in a new header field.

That way, the integrator can use the message 2 for the commit log message
and is free to typofix it, without breaking later third-party audit which
would use what is taken literally from the signed tag and stored in the
new header field, because the integrator's editor would never touch the
latter.