cassandra/cql: Fix snapshot_branch_get behavior with exclude prefix
When branches_from
parameter was greater than branch_name_exclude_prefix
,
branches whose name is greater than the prefix but lesser than branches_from
could be returned.
So do not call snapshot_branch_get_range
method for such cases and simply
call snapshot_branch_get_from_name
.
Related to swh/infra/sysadm-environment#4863 (closed)