Background
I have a repo that was duplicated, rather than forked, from another repo. This was done because the duplicated repo needs to be private whereas the parent repo is public, and Github does not permit a fork of a public repo to be made private.
(Note: the duplicate was created according to Github's docs here.
My goal
The duplicated repo will have independent work done on it. I wish to compare the parent repo with the duplicated one so that desired changes to the parent can be integrated with the duplicate. For convenience, I wish to use Github's /compare view rather than a local difftool.
The problem
Github's cross-repo /compare view appears to only work when the repos are forks of one another. The URL syntax indicated by Github's docs is:
https://github.com/user1/repo/compare/branch...user2:branch
When I try to use the same URL syntax with the duplicate to compare both master branches, I get the error There isn’t anything to compare.
The drop-down menus for head and base repo do not list the parent (and if I use /compare on the parent, do not list the duplicate) and are not editable. My guess is that this is because the duplicate was not forked, but duplicated.
Question
Is there a way to make the Github /compare view work with a duplicated repo, as opposed to a forked one? For example, is there a git config setting that can be changed to make it appear as a fork, an alternative URL syntax, etc etc.
Or alternatively, is there a way to accomplish my goal of comparing the parent and child repos if some other means of creating the child repo (as opposed to duplication) is used, with the proviso that the child must be able to be made private?
Additional note
I am able to successfully diff the repos locally via the remote config and commands in this question, but being able to compare on Github itself would be much better for my team.
All guidance is much appreciated!