Add create_branch option to force create a new branch

[stefanzweifel]

This PR is yet another attempt to solve the problem of "what should happen if the given branch does not exist yet".
As previous attempts failed and broke the action for all userrs, I've decided to add a new option called create_branch to make this basically an opt-in feature.

By default create_branch is false and using a branch-name that doesn't exist yet, fails the action with "fatal: invalid reference: not-existend-branch".

Setting create_branch to true, we use the -B-option of git-checkout to create a new branch. From there the Action works like before and pushes the commit to the remote branch.

Related Issues and PRs

• Create a branch if it doesn't exist #202
• Feature: Create a new branch, if the given branch name doesn't exist yet #140
• Creates new branch even if it exits #142