fix: branch planner plans against main instead of PR branch when using Flux Operator

[maruina]

The Branch planner's reconcileSource creates PR-specific GitRepository objects by deep-copying the original source spec and setting spec.Reference.Branch to the PR branch.

However, this doesn't work as expected when using a FluxInstance object: with a spec similar to

  sync:
    interval: 1m
    kind: GitRepository
    path: clusters/my-cluster
    pullSecret: flux-system
    ref: refs/heads/main
    url: https://github.com/my-org/my-repo.git

this gets translated into a GitRepository object with

spec:
  interval: 1m0s
  ref:
    name: refs/heads/main
  secretRef:
    name: flux-system
  timeout: 60s
  url: https://github.com/my-org/my-repo.git

FluxInstance.sync.ref: refs/heads/main is mapped to GitRepository.spec.ref.name: refs/heads/main and not spec.ref.branch.
According to the Gitrepository type, the order of precedence is: Name > Commit > SemVer > Tag > Branch

The problem is the branch planner's deep copy preserves that name field, which always wins over branch.
This causes PR plans to run against main instead of the PR branch.

This PR fix the issue by clearing those higher precedence field and setting just the branch