fix: Make netlify-fetch immune to stale cached release branches

Production builds failed with antora unable to find a commit on
release-26.7: the branch had been force-pushed after creation, the old
tip only survived as a local ref in the netlify build cache, and the
loop's ref update had failed silently (background auto-gc can race the
ref update; -q swallowed the error and the loop continued).

- Branch list from git ls-remote instead of the cached remote-tracking
  refs (which make expanded before the fetch even ran, so a brand-new
  release branch was invisible to its first build).
- gc.auto=0 inside the loop, no more silent failures: set -e aborts
  the build at the failing command instead of surfacing as a
  missing-object error from antora much later.
- git fetch --all --prune keeps the remote-tracking refs honest.

Author: lfrancke

Makefile

@@ -63,11 +63,16 @@ netlify-fetch:
 	# netlify messes with some files, restore everything to how it was:
 	git reset --hard
 	# fetch, because netlify does caching and we want to get the latest commits
-	git fetch --all
-	for remote in $(shell git branch -r | grep -E 'release[/-]'); do \
-		branch="$${remote#origin/}"; \
-		git fetch -q origin "+refs/heads/$$branch:refs/heads/$$branch"; \
-		git checkout -q "$$branch" -- ':(exclude)ui' . ; \
+	git fetch --all --prune
+	# The branch list comes from the remote so it is current even on the first
+	# build after a new release branch appears (the cached remote-tracking refs
+	# lag behind). Auto-gc is disabled inside the loop: a background gc can
+	# race the ref updates and silently leave a stale local branch behind,
+	# which only surfaces much later as a missing-object error from antora.
+	# Failures abort the build immediately instead.
+	set -e; for branch in $$(git ls-remote --heads origin | sed 's|.*refs/heads/||' | grep -E 'release[/-]'); do \
+		git -c gc.auto=0 fetch origin "+refs/heads/$$branch:refs/heads/$$branch"; \
+		git -c gc.auto=0 checkout -q "$$branch" -- ':(exclude)ui' . ; \
 	done
 	# restore the working tree of the current commit and drop files that
 	# only exist on other branches (ignored files like node_modules stay)