chore: document cherrypick.sh and use HEAD instead of main

Author: ARR4N

libevm/tooling/release/cherrypick.sh

@@ -1,21 +1,47 @@
 #!/usr/bin/env bash
 
+#  Copyright 2025 the libevm authors.
+#
+#  The libevm additions to go-ethereum are free software: you can redistribute
+#  them and/or modify them under the terms of the GNU Lesser General Public License
+#  as published by the Free Software Foundation, either version 3 of the License,
+#  or (at your option) any later version.
+#
+#  The libevm additions are distributed in the hope that they will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
+#  General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public License
+#  along with the go-ethereum library. If not, see
+#  <http://www.gnu.org/licenses/>.
+
+# Usage: run `./cherrypick.sh` on a branch intended to become a release.
+#
+# Reads the contents of ./cherrypicks, filters out commits that are already
+# ancestors of HEAD, and calls `git cherry-pick` with the remaining commit
+# hashes.
+
 set -eu;
 set -o pipefail;
 
 SELF_DIR=$(dirname $0)
 # The format of the `cherrypicks` file is guaranteed by a test so we can use simple parsing here.
 CHERRY_PICKS=$(cat "${SELF_DIR}/cherrypicks" | grep -Pv "^#" | awk '{print $1}')
-RELEASE_REF="main";
 
 commits=()
 for commit in ${CHERRY_PICKS}; do
-    git merge-base --is-ancestor "${commit}" "${RELEASE_REF}" && \
+    git merge-base --is-ancestor "${commit}" HEAD && \
         echo "Skipping ${commit} already in history" && \
         continue;
 
     echo "Cherry-picking ${commit}";
     commits+=("${commit}");
 done
 
+if [[ -z "${commits[@]// }" ]]; then # $x// removes whitespace
+    echo "No commits to cherry-pick";
+    exit 0;
+fi
+
 git cherry-pick "${commits[@]}";