merge-ort: store messages in a list, not in a single strbuf

merge-ort: store messages in a list, not in a single strbuf

To prepare for using the `merge-ort` machinery in server operations, we
cannot simply produce a free-form string that combines a variable-length
list of messages.

Instead, we need to list them one by one. The natural fit for this is a
`string_list`.

We will subsequently add even more information in the `util` attribute
of the string list items.

Based-on-a-patch-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>