sequencer: add newline before adding footers

sequencer: add newline before adding footers

When encountering a commit message that does not end in a newline,
sequencer does not complete the line before determining if a blank line
should be added.  This causes the "(cherry picked..." and sign-off lines
to sometimes appear on the same line as the last line of the commit
message.

This behavior was introduced by commit 967dfd4 ("sequencer: use
trailer's trailer layout", 2016-11-29). However, a revert of that commit
would not resolve this issue completely: prior to that commit, a
conforming footer was deemed to be non-conforming by
has_conforming_footer() if there was no terminating newline, resulting
in both conforming and non-conforming footers being treated the same
when they should not be.

Resolve this issue, both for conforming and non-conforming footers, and
in both do_pick_commit() and append_signoff(), by always adding a
newline to the commit message if it does not end in one before checking
the footer for conformity.

Reported-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>