Jeff King [off-list ref] writes:

All that said, if the main goal is just avoiding accidental diffs in
commit messages (and not worrying about truncation due to "---" in
messages), there may be a simpler receiver-side solution. If the
receiver expects the message to be generated by Git, then it will expect
there to be a "---" line. And we will not expect any diff before then.
So could we just have a "git am --strict" mode (and perhaps matching
config option) that always looks for the "---" separator?

It's not foolproof, but I suspect it would help with the worst cases of
embedded diffs. And it's not that hard to implement.

Yup, if the payload is known to be generated by "git", which is much
more likely these days than back when "git am" (or "git applymbox")
was written, can ignore "Index:" and "diff -" when deciding when the
log message part of a piece of e-mail ends.  I like that approach.