Keep track of parent repo

[thatch]

For illustration, my projects look like this

https://github.com/python-packaging/skel

https://github.com/thatch/imperfect (branches main, skel)
local clone (main, skel) referring to (origin/main, origin/skel)

Those two github urls do not share any common ancestors; the repo skel has the code and templates to generate the branch skel. If someone wants a different default license, or changes to defaults (like whether to use codecov.io), they fork that repo.

Towards and eventual goal of e.g. a skelup command/service that runs regen-git for you against a checkout of the appropriate repo skel somewhere, we need to know what the appropriate one is. When running regen-git for the first time (after this change), we can look at that repo skel checkout's origin and record that as the default parent.

Storing this in .vars.ini in a new section seems reasonable.