sequencer.c: require a conforming footer to be preceded by a blank line

sequencer.c: require a conforming footer to be preceded by a blank line

Currently, append_signoff() performs a search for the last line of the
commit buffer by searching back from the end until it hits a newline.  If
it reaches the beginning of the buffer without finding a newline, that
means either the commit message was empty, or there was only one line in it.
In this case, append_signoff will skip the call to has_conforming_footer
since it already knows that it is necessary to append a newline before
appending the sob.

Let's perform this function inside of has_conforming_footer where it
appropriately belongs and generalize it so that we require that the
footer paragraph be an actual distinct paragraph separated by a blank
line.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>