Single patch and cover letter: inconsistent Subject line behavior

[Marc Herbert <marc.herbert@linux.intel.com>]

The current documentation and cover-letter template state this:

> When you only have a single patch, b4 should "mix-in" the contents of
> the cover letter into the "under-the-cut" portion of the patch itself,
> where it serves as a source of additional information for the reviewers,
> but never makes it into the actual commit.

I noticed that the "under-the-cut" portion omits the _subject line_ of
the cover letter. That does not seem documented. So I wrote a
documentation patch and I was about to send it. But after testing a bit
more I noticed that the first and second lines of the cover letter _are_
included in the "under-the-cut" portion when the second line of the
cover letter file is not empty. Is that the intended behavior? Or maybe
more "accidental" and caused by whatever library parses the cover letter
file? Either way there is at least some documentation gap.


Correct subject line followed by empty line: OMITTED
   < empty line >
Body or not, included if any
Body or not, included if any


"Malformed subject" 1st line NOT followed by empty line: INCLUDED
   NOT empty 2nd line, INCLUDED
Body or not, included if any
Body or not, included if any