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

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(): simplify NOT_A_REPO error message

diff --cc setup.c
Simple merge

diff --cc setup.h
Simple merge

diff --cc submodule.c
Simple merge

diff --cc t/t7450-bad-git-dotfiles.sh
Simple merge