When we added caching, this meant that calling `can_be_resolved_in_ui?` didn't
always call `lines`, which meant that we didn't get the benefit of the
side-effect from that, where it forced the conflict data itself to UTF-8.

To fix that, make this explicit by separating the `raw_content` (any encoding)
from the `content` (which is either UTF-8, or an exception is raised).