Otherwise an empty subtree would be added to the parent tree.

If the stored tree contained an empty subtree, simplify() wouldn't work
against new "absent" subtree representation. I don't know if there's a
such code path, but I believe it's very rare to encounter the problem.

jj-vcs#2654

…flict

I think this is a variant of the problem fixed by 7fda80f "tree: simplify
conflict before resolving at hunk level." We need to simplify() the conflict
before and after extracting file ids because the source conflict values may
contain trees to be cancelled out, and the file values may differ only in exec
bits. Since the legacy tree passes a simplified conflict in to this function,
I made the merged tree do the same.

Fixes jj-vcs#2654