Replace deprecated `exists?` for ruby 3.2+

[markbluhm]

exists? deprecated in 2.2 and removed in 3.2,

can't push a PR but just need to swap exists? with exist?

git-safe-ruby/lib/git-safe/git.rb

Line 164 in 7137915

File.exists?(git_config_path)

https://github.com/perryqh/git-safe-ruby/blob/master/lib/git-safe/git.rb#L172

git-safe-ruby/spec/git-safe/git_spec.rb

Line 149 in 7137915

expect(Dir.exists?(File.join('.git'))).to be_truthy