Support README.md being a symbolic link
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### Summary
When `README.md` is a symlink to another file (say `README`), Gitlab web UI should render the contents of the linked file, not just the name of the linked file.
### Steps to reproduce
* Create a file `README` in markdown format.
* Create `README.md` as a symlink to `README` so that Gitlab picks it up for pretty rendering: `ln -s README README.md`
* Push the 2 files to the remote.
### Example Project
Someone is proposing a MR to GIMP to have its README as markdown: https://gitlab.gnome.org/GNOME/gimp/-/merge_requests/193
I still consider the README as the main first file to be read in a project, because pure text is what matters when one develops (you usually develop in some terminal or code editor, not on the web). Still considering that Markdown is pretty-much text-like, it's ok to use it as the README format if people really care about reading this file in a web browser (why not).
Yet we still need README because it's a GNU project (having a README is a GNU base concept and it makes sense). I know we could make the opposite symlink (`README` could be a symlink to `README.md`) but it would be really nice is Gitlab could properly support symbolic links, which are like a base feature of our systems since forever. ;-)
Also as said, I really prefer `README` to be conceptually the source, not the other way around.
### What is the current *bug* behavior?
The name of the target file is displayed only, i.e. "README"
### What is the expected *correct* behavior?
The contents of `README` should be pretty-rendered on the Gitlab page.
### Relevant logs and/or screenshots
How the README.md is rendered when it's a symlink to README:

### Output of checks
This bug happens on gitlab.gnome.org, I have not tested on other instances.
#### Results of GitLab environment info
…
#### Results of GitLab application Check
…
### Possible fixes
…
issue