Mark superseded commits honestly instead of skipped (closes #152)

Gitea cancels an in-flight run when a newer commit lands on the same
branch and records the cancellation as `failure` / "Has been
cancelled". The workflow rewrote that to `skipped`, but Gitea's
Combine() folds `skipped` into `success`, so the combined-status API
returned green for a commit nothing had ever tested. Rewrite it to
`failure` / "Superseded by a newer commit; never tested" instead:
red-but-honest, and never `pending`, which would block the commit
forever.

Re-running the superseded commit would have been better still, but is
not reachable on this Gitea (1.25.4): its API exposes no rerun
endpoint, workflow dispatch takes a ref rather than a SHA, and every
replay would be a full uncached build with no bound on how many pile
up behind a burst of merges.

The step also stops hardcoding its status context: the logic moves into
script/ci-mark-superseded, which derives the context from the workflow
name, job id and event, and fails loudly when no status on the commit
being built carries that context, so renaming the workflow or the job
cannot silently disable the rewrite. The derivation is not byte-exact
with Gitea's own rule -- Gitea uses the job's display `name:` where the
runner exports the job id -- so adding a `name:` to the job turns every
push red rather than quietly doing nothing; the script header says so,
because that loud failure is the point. That is item 2 of
#147; item 1 there is
untouched.

Nothing about the walk may fail quietly, since the script exists to
stop CI lying quietly. An ANCESTOR_LIMIT that is set but not a positive
integer aborts instead of passing an unusable value to git and
discarding the error; the root-commit case is detected explicitly so
every other rev-list failure aborts too; and per-ancestor status reads
carry the same `--retry 3 --max-time 30` as the head-commit read and
abort on failure rather than losing curl's exit status through a pipe.

Tests drive the script against a fake Gitea covering the cancelled,
laundered-skipped, genuinely-failed, passing and renamed cases, an
unparseable ANCESTOR_LIMIT and an ancestor whose status read answers
HTTP 500, so jq joins the builder image to run them.

.gitea/workflows/check.yml

@@ -13,39 +13,19 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2 2024-10-23
         with:
           # The fingerprint step below needs history to find the last commit
-          # that touched the Docker build context.
+          # that touched the Docker build context, and the superseded-status
+          # step needs it to walk ancestors (it aborts on a shallow clone).
           fetch-depth: 0
 
-      - name: Neutralize superseded run statuses
+      - name: Mark superseded run statuses
         # Gitea cancels the in-flight run when another commit is pushed to the
         # same branch and records the cancellation as `failure`, so a commit
-        # that was never tested reads red. The cancellation is unconditional
-        # server-side for push events and cannot be disabled from a workflow
-        # file, so the superseding run rewrites those statuses to `skipped`.
-        # Only the exact cancellation status is touched; a real failure is
-        # left alone.
+        # that was never tested reads as a test result. The script rewrites
+        # those statuses to say what happened. See its header for why the
+        # state stays `failure` and not `skipped`.
         env:
           GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
-        run: |
-          set -eu
-          api="${GITHUB_API_URL}/repos/${GITHUB_REPOSITORY}"
-          ctx='check / check (push)'
-          for sha in $(git rev-list --max-count=20 "${GITHUB_SHA}^" || true); do
-            latest="$(curl -sf "${api}/commits/${sha}/status" | jq -r \
-              --arg c "$ctx" \
-              '[.statuses[] | select(.context == $c)][0] // empty
-               | "\(.status)|\(.description)"')" || continue
-            [ "$latest" = 'failure|Has been cancelled' ] || continue
-            curl -sf -X POST "${api}/statuses/${sha}" \
-              -H "Authorization: token ${GITEA_TOKEN}" \
-              -H 'Content-Type: application/json' \
-              -d "$(jq -nc --arg c "$ctx" '{
-                context: $c,
-                state: "skipped",
-                description: "Superseded by a newer commit; never tested"
-              }')" >/dev/null
-            echo "neutralized superseded status on ${sha}"
-          done
+        run: script/ci-mark-superseded
 
       - name: Fingerprint the build context
         # `.dockerignore` keeps docs out of the build context, so a docs-only