Remember to attach the parent tree when converting TreeEntry() -> Tree() (#22902)

Remember to attach the parent tree when converting TreeEntry() -> Tree() (#22902)

!fixup https://github.com/go-gitea/gitea/pull/22177

The only place this function is used so far is in
findReadmeFileInEntries(), so the only visible effect of this oversight
was in an obscure README-related corner: if the README was in a
subfolder and was a symlink that pointed up, as in .github/README.md ->
../docs/old/setup.md, the README would fail to render when FollowLinks()
hit the nil ptree. This makes the ptree non-nil and thus repairs it.