Don't add ref to node graph if it already exists

[tranhl]

Adding a ref (e.g. main) to the branch graph will cause a runtime error:

Run git-town/action@v1
/home/runner/work/_actions/git-town/action/v1/dist/index.js:2[4](https://github.com/connecX/monorepo/actions/runs/8963195676/job/24613133789?pr=557#step:3:5)845
        throw new UsageGraphError('Graph.addNode: the "'.concat(node2, '" node already exist in the graph.'));
              ^

UsageGraphError2: Graph.addNode: the "main" node already exist in the graph.
    at _addNode (/home/runner/work/_actions/git-town/action/v1/dist/index.js:2484[5](https://github.com/connecX/monorepo/actions/runs/8963195676/job/24613133789?pr=557#step:3:6):15)
    at MultiDirectedGraph3.addNode (/home/runner/work/_actions/git-town/action/v1/dist/index.js:25574:24)
    at /home/runner/work/_actions/git-town/action/v1/dist/index.js:42854:15
    at Array.forEach (<anonymous>)
    at main (/home/runner/work/_actions/git-town/action/v1/dist/index.js:42853:1[6](https://github.com/connecX/monorepo/actions/runs/8963195676/job/24613133789?pr=557#step:3:7))
    at run (/home/runner/work/_actions/git-town/action/v1/dist/index.js:46955:10)
    at process.processTicksAndRejections (node:internal/process/task_queues:[9](https://github.com/connecX/monorepo/actions/runs/8963195676/job/24613133789?pr=557#step:3:10)5:5)

We should check whether the ref exists as a node in the graph first before adding it.