setup_applied does `git init --initial-branch=main` then
`git fetch <proposed> main:refs/heads/main` to seed the
applied repo with proposed's initial commit. git's default
safeguard refuses to fetch into the currently-checked-out
branch, even though the working tree is empty (we just init'd).
add --update-head-ok to bypass — the read-tree-reset
immediately after fetches the right state, so the safeguard
the flag bypasses isn't relevant here anyway.
repro from the user: spawn of 'dmatrix' failed with
fatal: refusing to fetch into branch 'refs/heads/main'
checked out at '/var/lib/hyperhive/applied/dmatrix'
c92108a11c