support relative links from wikipages to files in its project's repo (not just its project's Downloads) (BB-13553)

Thanks for fixing issue#=6315 and its duplicates (e.g., issue#=6589. However, as noted here, those issues address relative-linking from one file (e.g., a README) to another (e.g., an image) within a given project's repository. That's useful, but IMHO It Would Be Better to also to support relative links from a wikipage to a file in the project's repository. E.g., suppose

1. One has a project with name=project_name rooted @ .../project_name/
2. The project's owner creates a readme visible as project overview at the canonical location=.../project_name/README
3. The project's owner creates a wiki at the canonical location=.../project_name/wiki/
4. The project's owner creates a folder/subdir @ .../project_name/images/ and commits files to the project repo in that folder/subdir.

Currently (i.e., since the fix to issue#=6315 and its duplicates), the following relative-link syntaxes (doubtless et al) work (as demonstrated by my project=rst_link_test:

1. from .../project_name/README to .../project_name/images/image_file with the following syntax:

1. 1. /images/image_file
   2. ./images/image_file
   3. images/image_file
2. from .../project_name/wiki/Home to an image_file in the project's Downloads with the (sole) syntax ../downloads/Jen_Sorensen_PandP-poster-ForWeb.png as demonstrated here

I realize that you currently intend to keep wiki repositories separate from their project's repo. That being said, the currently-supported behavior seems to demonstrate that there is no technical impediment to a wikipage relative-linking to a file in its project's repo.