I think the most natural syntax (and the one I tried before discovering it didn't work!) would be to allow one to basically copy-paste the existing path at the top of file pages (the one inside the phui-header-header element), so that for example, a link to Diffusion's readme file on here would be: rP/README.md
And to link to, say, a specific line of a specific revision: rPbe0e84a81abc/README.md$17
And likewise for ranges: rPbe0e84a81abc/README.md$5-13