gha: Manually rebase PR atop of the target branch before testing

We're changing what's been done as part of ac939c458c, as we've
notcied issues using `github.event.pull_request.merge_commit_sha`.

Basically, whenever a force-push would happen, the reference of
merge_commit_sha wouldn't be updated, leading us to test PRs with the
old code. :-/

In order to get the rebase properly working, we need to ensure we pull
the hash of the commit as part of checkout action, and ensure
fetch-depth is set to 0.

Fixes: #7414

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

tests/git-helper.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2023 Intel Corporation
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+function add_kata_bot_info() {
+	echo "Adding user name and email to the local git repo"
+
+	git config user.email "katacontainersbot@gmail.com"
+	git config user.name "Kata Containers Bot"
+}
+
+function rebase_atop_of_the_latest_target_branch() {
+	if [ -n "${TARGET_BRANCH}" ]; then
+		echo "Rebasing atop of the latest ${TARGET_BRANCH}"
+		git rebase origin/${TARGET_BRANCH}
+	fi
+}
+
+function main() {
+    action="${1:-}"
+
+    add_kata_bot_info
+
+    case "${action}" in
+	rebase-atop-of-the-latest-target-branch) rebase_atop_of_the_latest_target_branch;;
+        *) >&2 echo "Invalid argument"; exit 2 ;;
+    esac
+}
+
+main "$@"