RelNotes: replace Gmane with real Message-IDs

RelNotes: replace Gmane with real Message-IDs

The only references to Gmane that remain are in RelNotes. Although these
are definitely not in active use, they might be of historical interest
for future readers so let's ensure that mail references are more robust.

Replace links to Gmane with links to lore.kernel.org (which is our new
preferred mailing list archive and has the Message-ID in the URL) and
bare Gmane ID references with Message-IDs.

The Message-IDs were found by searching for "gmane:<id>" on
https://public-inbox.org/git/ and taking the resulting message.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>