This PR updates the _push_to_github-function and the call to git push to make the branch input option optional.

If no branch name has been given, we now just execute git push origin without an explicit branch name.

I've tested this with actions/checkouts@v2 and with Workflows listening to the push event, and it works great.

• Workflow Run: https://github.com/stefanzweifel/git-auto-commit-action-demo-app/runs/430150745
• Workflow File: https://github.com/stefanzweifel/git-auto-commit-action-demo-app/blob/e062ad8e5b0c7087808313c9930240c8987cba95/.github/workflows/format_php.yml
• Created Commit: stefanzweifel/git-auto-commit-action-demo-app@60d40ed

Closes #27