Add test for auto-forwarding a branch checked out in another worktree

When the linked worktree's branch is changed externally — by another
shell, by another tool, or by git running outside lazygit — lazygit's
worktrees model goes stale. The next post-fetch auto-forward then
doesn't realise the branch is now checked out elsewhere, and advances
its ref behind the worktree's back. The worktree's HEAD then resolves
to a commit its index/working tree haven't been updated to, and the
user sees that diff as the inverse of what was fetched — files
appearing as pending changes that they didn't make.

The test sets up a linked worktree initially on a side branch, then
externally checks out master in it before pressing fetch. Two
EXPECTED/ACTUAL pairs capture the symptoms: the branches view shows
master as `✓` rather than `↓1`, and switching to the linked worktree
shows master's would-be incoming file as a pending deletion against
HEAD.

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