Use correct README link to render the README (#23152)

`renderReadmeFile` needs `readmeTreelink` as parameter but gets
`treeLink`.
The values of them look like as following:
`treeLink`:  `/{OwnerName}/{RepoName}/src/branch/{BranchName}`
`readmeTreelink`:
`/{OwnerName}/{RepoName}/src/branch/{BranchName}/{ReadmeFileName}`

`path.Dir` in

8540fc45b1/routers/web/repo/view.go (L316)
should convert `readmeTreelink` into
`/{OwnerName}/{RepoName}/src/branch/{BranchName}` instead of the current
`/{OwnerName}/{RepoName}/src/branch`.

Fixes #23151

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

models/fixtures/repository.yml

@@ -25,7 +25,7 @@
   fork_id: 0
   is_template: false
   template_id: 0
-  size: 6708
+  size: 7028
   is_fsck_enabled: true
   close_issues_via_commit_in_any_branch: false