release.sh: refuse to pin from a non-master branch (the pin commit lands on master)

release.sh

@@ -49,6 +49,10 @@ cmd_build() {
 
 cmd_pin() {
   local tag="${1:-}" ; [ -n "$tag" ] || die "usage: release.sh pin <tag>"
+  local br; br=$(git rev-parse --abbrev-ref HEAD)
+  if [ "$br" != "master" ] && [ "${ALLOW_BRANCH:-0}" != "1" ]; then
+    die "on branch '$br' — the pin commit goes to master, so merge your change first (or set ALLOW_BRANCH=1 if you really mean it)"
+  fi
   python3 - "$tag" <<'PY'
 import json, sys
 tag = sys.argv[1]
@@ -61,7 +65,7 @@ print(f'dockerImage: {old} -> {m["dockerImage"]}')
 PY
   git add "$MANIFEST"
   git commit -q -m "Pin manifest to $tag"
-  git push -q origin HEAD:master
+  git push -q
   ok "pinned + pushed $tag"
 }