Merge branch 'ty/setup-error-tightening'

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