sequencer: ignore "---" divider when parsing trailers

sequencer: ignore "---" divider when parsing trailers

When the sequencer code appends a signoff or cherry-pick
origin, it uses the default trailer-parsing options, which
treat "---" as the end of the commit message. As a result,
it may be fooled by a commit message that contains that
string and fail to find the existing trailer block. Even
more confusing, the actual append code does not know about
"---", and always appends to the end of the string. This can
lead to bizarre results. E.g., appending a signoff to a
commit message like this:

  subject

  body
  ---
  these dashes confuse the parser!

  Signed-off-by: A
results in output with a final block like:

  Signed-off-by: A
  Signed-off-by: A
The parser thinks the final line of the message is "body",
and ignores everything else, claiming there are no trailers.
So we output an extra newline separator (wrong) and add a
duplicate signoff (also wrong).

Since we know we are feeding a pure commit message, we can
simply tell the parser to ignore the "---" divider.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>