notes.c: append separator instead of insert by pos

notes.c: append separator instead of insert by pos

Rename "insert_separator" to "append_separator" and also remove the
"postion" argument, this serves two purpose:

The first is that when specifying more than one "-m" ( like "-F", etc)
to "git notes add" or "git notes append", the order of them matters,
which means we need to append the each separator and message in turn,
so we don't have to make the caller specify the position, the "append"
operation is enough and clear.

The second is that when we execute the "git notes append" subcommand,
we need to combine the "prev_note" and "current_note" to get the
final result. Before, we inserted a newline character at the beginning
of "current_note". Now, we will append a newline to the end of
"prev_note" instead, this will give the consisitent results.

Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>