Merge branch 'ty/setup-error-tightening'

While discovering a ".git" directory, the code treats any stat() failure as a sign that a filesystem entity .git does not exist there, and ignores ".git" that is not a "gitdir" file or a directory. The code has been tightened to notice and report filesystem corruption better. * ty/setup-error-tightening: setup: improve error diagnosis for invalid .git files