Quote "Sub" and "Main" to emphasize they're made up

Thanks @toulouse for pointing out that's not clear

More should probably be done to emphasize that, maybe commas to induce a
pause like (let's call one, "Sub").

Recommend --word-diff-regex=. to view diff.
Comparison with git-subtree mentions started out as fork
s/the superproject/Main/ #tinyedit

More consistent and concise to just say "Main" instead of "the
superproject" everywhere after the intro.

Also re-word-wrap, hence:

Recommend --word-diff-regex=. to view diff.