Don't assume upstream on branch creation in graph

[justinrobots]

When creating a new branch in the graph by right-clicking a branch ref, GitLens currently assumes that you want to set the upstream of the newly-created branch to the branch you're creating it from and does it without asking.

This has caused issues multiple times when I push changes because they go to main instead of my-feature-branch, triggering builds and updating release notes in production.

originates from this slack conversation

[sergeibbb]

Hi @justinrobots This describes the current behavior but it does not explore the desired changes.

[eamodio]

We will now do 2 things:

• Avoid pre-filling the input box with the branch name when the name already exists (e.g. choosing a remote branch, where a local branch with the same name exists)
• If you create a new branch from a remote branch, only add tracking if the name of the new branch matches the remote branch.