fix(unmerge): Fix message field in group unmerge (#14897)
fix(unmerge): Fix message field in group unmerge (#14897)

Fetch the message field from Snuba to make sure we have have a correctly
formatted message to update the group with. The message in node data might
not be present or incorrectly formatted (it's a dict and not text).
beeftornado/sentry,beeftornado/sentry,beeftornado/sentry