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

[kousu]

!fixup #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.

[kousu]

For example, with this repo:

sdffsf$ tree -a -I .git
.
├── abcdef
│   └── 13423423
│       └── README.md
└── .github
    └── README.md -> ../abcdef/13423423/README.md

3 directories, 2 files

Then before #22177 (b0f1872):

With #22177 (e1aca7c):

But after this (df33253) it's fixed:

[lafriks]

please update with base branch

[zeripath]

make lgtm done