sync: allow 3-way merge for patching to simplify manual conflict resolution

Allowing --3way leaves conflicts in the local files, which makes manual
conflict resolution so much easier.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>

scripts/sync-kernel.sh

@@ -237,7 +237,7 @@ cd_to ${LIBBPF_REPO}
 git checkout -b ${LIBBPF_SYNC_TAG}
 
 for patch in $(ls -1 ${TMP_DIR}/patches | tail -n +2); do
-	if ! git am --committer-date-is-author-date "${TMP_DIR}/patches/${patch}"; then
+	if ! git am --3way --committer-date-is-author-date "${TMP_DIR}/patches/${patch}"; then
 		read -p "Applying ${TMP_DIR}/patches/${patch} failed, please resolve manually and press <return> to proceed..."
 	fi
 done