Support relative links from a wiki page to files in the repository

• Related to #3275 (closed)

Problem

• There's no way to create a relative link from a wiki page to a file in the same repository
• Previously, links of the form [README.md](../blob/master/README.md) from the wiki worked as expected
• This was a side effect of a lax implementation and was incidentally removed in #18019 (closed) / !4432 (merged)

Proposal

• Allow linking to repository files from a wiki page using syntax like [README.md](../blob/master/README.md)