gistfile1.txt

grahamc@Morbo>                                                                                                                                                            ~
awk -F: '...thatscript...' '/home/grahamc/.mail/grahamc/[Gmail]/All Mail/cur/...:2,S'

List-Unsubscribe: <mailto:unsub+...@reply.github.com>,
 <https://github.com/notifications/unsubscribe/...>

parse.awk

  # Set "unsub" to 0 indicating we have not hit an unsubscribe header
  # As soon as we reach one, we set it to 1 to indicate the next line
  # might be a header-folded unsubscribe line continuation
  begin { unsub=0 }

  # Abort if we find a line with no content, indicating going from
  # headers to body
  $0 == "" { exit }

  # If unsub=1 it means the previous line was a list-unsubscribe
  # header line. If the current line starts with a space, it is a
  # header continuation, so print it.
  $0 ~ /^\s+/ && unsub==1 { print $0 }
  
  # If unsub=1 it means the previous line was a list-unsubscribe
  # header line. Since the current line does not start with a space,
  # it is not a header continuation, so we've finished finding the
  # unsubscribe header.
  $0 !~ /^\s+/ && unsub==1 { exit }

  # We have reached a list-unsubscribe header, so set unsub=1 to
  # handle a header-folded line continuation and print the header
  tolower($1) == "list-unsubscribe" { unsub=1; print $0; }