GitHub mirrors contain a description under the main header such as
Github mirror of "performance/WebPageTest" - our actual code is hosted with Gerrit (please see https://www.mediawiki.org/wiki/Developer_access for contributing
This is very unhelpful, because the URL of the original repo has to be constructed manually. Typically, at this point I give up and just content myself with whatever I can see and do in the GitHub mirror.
The description should be something like
GitHub mirror of https://gerrit.wikimedia.org/r/#/admin/projects/performance/WebPageTest – please see https://www.mediawiki.org/wiki/Developer_access for contributing. Thanks!
or the equivalent Diffusion URL if T110607 allows.