bootstrap.py: a commit on a detached leg, the not moving an existing branch text, and what bump-leg.py and AGENTS-shape.md still promise (one re-pin) #533

make bootstrap leaves a commit made on a detached leg on no branch, and says nothing, when the leg's branch is already at the pin.

• The case:
  1. bump-leg.py --to the leg branch's own tip, so the branch is at the new pin.
  2. Someone makes a commit on the leg while it is detached there.
  3. make bootstrap runs.
• place_at_pin (templates/assembly-root/scripts/bootstrap.py:255-272) runs the checkout of the branch and prints on main at <pin> (branch tip == pin).
• The commit is now on no branch and only HEAD's reflog holds it. The review's run: 23bfef45a582 was on 0 branches, with 2 reflog entries, and nothing was printed about it.
• fast_forward_to_pin guards exactly this case. Its docstring (:296-299) says "checking the branch out would leave it only in the reflog". It refuses with "…the checkout is at (detached), which is neither" (:313-322). place_at_pin has no such guard. 7348c57b^, before templates/assembly-root/scripts/bootstrap.py: a tracking branch behind the pin is fast-forwarded to it, and a fresh clone's branch past the pin with no commit of its own is moved back to it, so bump-leg.py then make bootstrap, and git clone --recurse-submodules then make bootstrap, leave the leg on its branch AT the pin, no longer detached under not moving an existing branch (#475) #490, behaves the same.
• Possible fix: when HEAD is detached at a commit other than the pin, place_at_pin refuses as fast_forward_to_pin does and leaves the leg where it is.

— #458, comment

bootstrap's not moving an existing branch line says "The checkout stays at the pin", which is false when HEAD is on that branch.

• The line is at templates/assembly-root/scripts/bootstrap.py:446-448.
• tests/test_bootstrap_after_bump.py::test_a_branch_ahead_of_the_pin_is_not_moved (:239) asserts that HEAD stays on main at the ahead tip, not at the pin.
• Right after a bump the leg is detached at the pin, so the clause is true in that case only.
• Possible fix: name where the checkout is, as the "which is neither" line does.

— #458, comment

Since #490 (7348c57), make bootstrap puts a leg back on its tracking branch only when that branch is at or behind the pin, or ahead of it with no commit of its own; a branch with commits of its own past the pin is not moved, and the leg stays detached at the pin. Three texts still promise more: AGENTS.md:355 and README.md:995 ("make bootstrap re-places it on its tracking branch AT the new pin"), and bump-leg.py's unconditional print after every bump (scripts/bump-leg.py:1513). Every project's AGENTS-shape.md and docs/handbook.html carry the same promise. Lane openRepoShape-5's suggested words, from its read of #490: "make bootstrap in the root puts it back on its tracking branch when that branch is at or behind the new commit, or ahead of it with no commit of its own; a branch with commits of its own past the pin is not moved (bootstrap says not moving an existing branch), and the leg stays detached at the pin. Say which when you relay the result." Stale the other way: shape-doctor.py:2914 and :2989 and tests/test_doctor_legs_members_next.py:10 still say bootstrap never moves an existing branch whose tip is not the pin. From lane openRepoShape-5's read of #490 and the review of #490.
— #458, body

bootstrap prints checked out, and then git said: (templates/assembly-root/scripts/bootstrap.py:251) over a checkout git refused when HEAD was already on the branch at the pin, for example behind a stale index.lock. Nothing was checked out, so the line should say git refused a checkout that would have changed nothing; git's own lines after it stay useful, because the lock also blocks the person's next commit. Nit 1 of the verification of #490.
— #458, body