notes.c: Use two newlines (instead of one) when concatenating notes

notes.c: Use two newlines (instead of one) when concatenating notes

When using combine_notes_concatenate() to concatenate notes, it currently
ensures exactly one newline character between the given notes. However,
when using builtin/notes.c:create_note() to concatenate notes (e.g. by
'git notes append'), it adds a newline character to the trailing newline
of the preceding notes object, thus resulting in _two_ newlines (aka. a
blank line) separating contents of the two notes.

This patch brings combine_notes_concatenate() into consistency with
builtin/notes.c:create_note(), by ensuring exactly _two_ newline characters
between concatenated notes.

The patch also changes a few notes-related selftests accordingly.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>