Use baseBranch prefix whenever there's patterns

[rarkins]

Describe the proposed change(s).

Discussed in #34570

Quoting:

Consider the following config:

{
  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
  "dependencyDashboard": true,
  "baseBranches": [
    "$default",
    // Literal string
    "does-not-exist-yet",
  ],
}

The update branch name contains the default branch name after the branchPrefix, regardless of wether the other base branch exists. The dependency dashboard contains a warning (not mentioning the name) for the missing branch.

Compare this against:

{
  "dependencyDashboard": true,
  "baseBranches": [
    "$default",
    // Regular expression
    "/does-not-exist-yet/",
  ],
}

The dependency dashboard does not report an error, and the branch names for updates to the default branch does not contain the default branch. That is, until the creation of the "does-not-exist-yet" branch. Once it exists, all outstanding branches/PRs for the default branch will be deleted/autoclosed, and new branches (as configured) will be created with an infix of the branch name after the branchPrefix.

Change: we should add additionalBranchPrefix for all branches even if the pattern does not match anything.