Avoid auto-forwarding main branches checked out in other worktrees (#5621)

After fetching, we auto-forward main branches that have fallen behind
their upstream, but skip any that are currently checked out in another
worktree — otherwise we'd update the ref behind that worktree's back,
leaving its working copy showing the inverse of what was just fetched.

The skip check had a hole though: when starting lazygit while no
worktrees have a main branch checked out, leaving it running in the
background, and then checking out a main branch in one of the other
worktrees outside of lazygit (e.g. in a lazygit instance in another
terminal, or using `git checkout` in the shell, or using some other git
client or IDE), then lazygit wouldn't notice the change, and the next
fetch would auto-forward main even though it is now checked out in a
worktree.

The fix is to include `WORKTREES` in 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.

A few small things picked up along the way landed as separate commits
first:

- Preserve the empty-slice fallback in `loadWorktrees` when `git
worktree list` fails — the fallback was being overwritten by the nil
return value on the next line.
- Add `PULL_REQUESTS` to the manual fetch refresh scope to match the
background fetch; looks like an oversight from when PR support was
added.
- Extract `BranchesHelper.PostFetchRefresh` so the two fetch paths can't
drift again.

Fixes #5020.