ci/check-cherry-picks: improve error handling

We recently moved the $commits variable out of a "subshell in a
herestring", let's do the same for the list of branches, where errors
would be silently swallowed as well.

Also reformat the expressions slightly, we have enough line-length.

ci/check-cherry-picks.sh

@@ -14,9 +14,7 @@ cd "$(dirname "${BASH_SOURCE[0]}")"
 PICKABLE_BRANCHES=${PICKABLE_BRANCHES:-master staging release-??.?? staging-??.??}
 problem=0
 
-commits="$(git rev-list \
-  -E -i --grep="cherry.*[0-9a-f]{40}" --reverse \
-  "$1..$2")"
+commits="$(git rev-list -E -i --grep="cherry.*[0-9a-f]{40}" --reverse "$1..$2")"
 
 while read new_commit_sha ; do
   if [ -z "$new_commit_sha" ] ; then
@@ -45,6 +43,8 @@ while read new_commit_sha ; do
   for branch_pattern in $PICKABLE_BRANCHES ; do
     set +f # re-enable pathname expansion
 
+    branches="$(git for-each-ref --format="%(refname)" "refs/remotes/origin/$branch_pattern")"
+
     while read -r picked_branch ; do
       if git merge-base --is-ancestor "$original_commit_sha" "$picked_branch" ; then
         echo "  ✔ $original_commit_sha present in branch $picked_branch"
@@ -79,11 +79,7 @@ while read new_commit_sha ; do
         # move on to next commit
         continue 3
       fi
-    done <<< "$(
-      git for-each-ref \
-      --format="%(refname)" \
-      "refs/remotes/origin/$branch_pattern"
-    )"
+    done <<< "$branches"
   done
 
   if [ "$GITHUB_ACTIONS" = 'true' ] ; then