Merge branch 'jk/setup-gitfile-diag-fix' into seen

A regression in the error diagnosis code for invalid .git files has
been fixed, avoiding a potential NULL-pointer crash when reporting
that a .git file does not point to a valid repository.

* jk/setup-gitfile-diag-fix:
  read_gitfile_gently(): return non-repo path on error

diff --cc setup.c
Simple merge

diff --cc setup.h
Simple merge