Unify appending signoff in format-patch, commit and sequencer
There are two implementations of append_signoff in log-tree.c and
sequencer.c, which do more or less the same thing. Unify on top of the
sequencer.c implementation.
Add a test in t4014 to demonstrate support for non-s-o-b elements in the
commit footer provided by sequence.c:append_sob. Mark tests fixed as
appropriate.
[Commit message mostly stolen from Nguyễn Thái Ngọc Duy's original
unification patch]
Signed-off-by: Brandon Casey <bcasey@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>