git-commit: populate the edit buffer with 2 blank lines before s-o-b

'commit -s' populates the edit buffer with a blank line before the
Signed-off-by line, to allow the user to immediately start typing
the log message.  But commit 33f2f9ab removed this space, forcing
the user to first push the Signed-off-by line down to open a place
to type the log message.

Fix this regression and let's ensure that the Signed-off-by line is
preceded by two blank lines, instead of just one, to hint that
something should be filled in, and that a blank line should separate
it from the body and the Signed-off-by line.

Add a test for this behavior.

Reported-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>