"Master clone" named the repository after a branch it need not be on. An
adopted clone keeps whatever branch it had — verified: adopting a clone
checked out on a feature branch leaves it there — so the name was wrong for
every install that did not come from curl.
Renamed to "main clone" in dsh-upgrade and dsh-customize, describing its
actual role: the one real clone whose object store every worktree shares.
dsh-upgrade also now says not to assume the main clone sits on `master` or
that its `origin` is authoritative upstream, since an adopted clone may
point at a fork. The fetch itself was already correct: step 1 resolves
authoritative upstream separately, and step 4 fetches upstream `master`
from it rather than from the clone's own branch.
Turtle
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