sequencer: teach write_message() to append an optional LF

sequencer: teach write_message() to append an optional LF

This commit prepares for future callers that will have a pointer/length
to some text to be written that lacks an LF, yet an LF is desired.
Instead of requiring the caller to append an LF to the buffer (and
potentially allocate memory to do so), the write_message() function
learns to append an LF at the end of the file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>