Wiki doesn't handle "~" in page paths correctly

Summary

When using a ~ in a wiki page title, we create a ~ directory in the wiki repository and store the page there, but the ~ is expanded to the GitLab user's home directory /var/opt/gitlab in some situations.

Steps to reproduce

1. Create a wiki page with the title ~/test and some content.

What is the current bug behavior?

• In the wiki sidebar navigation, the page's directory gets displayed as var/opt/gitlab.
• Opening the page uses the expanded directory in the URL.
• When trying to delete the page, GitLab aborts with a 503 error.
• When trying to edit the page, GitLab responds with a validation error Someone edited the page the same time you did.

What is the expected correct behavior?

• We should never expand ~ into the home directory, and display it as a literal ~ instead.
• Editing and deleting a page with ~ in the path or title should work without errors.

Relevant logs and/or screenshots

Customer ticket: https://gitlab.zendesk.com/agent/tickets/148330

Wrong path in sidebar:

Output of checks

This bug happens on GitLab.com

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)