bug(git): a failing git worktree list (not a timeout) reads as 'not a repository'

[Juliusolsson05]

Found while verifying #1450. Make a repo with a linked worktree, then make its .git/worktrees/<name> folder unreadable. git worktree list fails outright rather than timing out, and the runner in src/main/ipc/git.ts turns every git failure into '', so the list is {worktrees: [], timedOut: false}. That is the same as 'not a repository', and resolveRepoRootAfterGit returns the worktree folder as the repository. This is the q126 false-repository result through a non-timeout path. Failures other than 'not a repository' should be reported as unknown, never as empty.

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