Refresh worktrees before auto-forwarding branches

AutoForwardBranches relies on the worktree model to skip any branch
that's currently checked out in another worktree (so we don't update
its ref behind the worktree's back). The post-fetch refresh wasn't
including the worktrees scope, so any external change to the worktree
list between lazygit's startup and the fetch — a `git worktree add`,
a `git checkout` in a linked worktree, a branch rename — left the
in-memory model stale and the skip check returned false negatives.

Add WORKTREES to the post-fetch refresh scope when auto-forwarding is
enabled. We gate on the config so users with auto-forward disabled
don't pay for an extra `git worktree list` plus per-worktree rev-parse
on every fetch tick.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>