`gemfile:` option causes git-hooks to fail if both Psych 4.x and 3.x are installed locally

[Drowze]

I'm not sure whether should I create this issue here or on the rubygems repository (to the Bundler maintainers). Let me know what you think.

A have overcommit installed with gemfile: Gemfile option on .overcommit.yml and most (all?) of my git-hooks have this script (this was removed from .git/hooks/post-checkout):

# Check if Overcommit should invoke a Bundler context for loading gems
require 'yaml'
# rubocop:disable Style/RescueModifier
if gemfile = YAML.load_file('.overcommit.yml')['gemfile'] rescue nil
  ENV['BUNDLE_GEMFILE'] = gemfile
  require 'bundler'

  begin
    Bundler.setup
  rescue Bundler::BundlerError => e
    puts "Problem loading '#{gemfile}': #{e.message}"
    puts "Try running:\nbundle install --gemfile=#{gemfile}" if e.is_a?(Bundler::GemNotFound)
    exit 78 # EX_CONFIG
  end
end
# rubocop:enable Style/RescueModifier

However, on my system when I execute that, it fails on Bundler.setup. See:

$ cat Gemfile
source 'https://rubygems.org'
gem 'psych', '~> 3.0'

$ bundle install
Using bundler 2.2.25
Using psych 3.3.2
Bundle complete! 1 Gemfile dependency, 2 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

$ gem list | grep 'yaml\|bundler\|psych'
bundler (default: 2.2.25, 2.2.16)
bundler-audit (0.8.0, 0.7.0.1)
pronto-bundler_audit (0.7.0, 0.6.0)
psych (4.0.1, 3.3.2, 3.3.1, default: 3.1.0)
yaml (0.1.1, default: 0.1.0)

$ irb
[1] pry(main)> require 'yaml'
=> true
[2] pry(main)> require 'bundler'
=> true
[3] pry(main)> Bundler.setup
Gem::LoadError: You have already activated psych 4.0.1, but your Gemfile requires psych 3.3.2. Prepending `bundle exec` to your command may solve this.
from /Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler/runtime.rb:309:in `check_for_activated_spec!'

On a real repository with Overcommit setup:

$ git checkout .
Updated 0 paths from the index
Traceback (most recent call last):
        7: from .git/hooks/post-checkout:37:in `<main>'
        6: from /Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler.rb:149:in `setup'
        5: from /Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler/runtime.rb:24:in `setup'
        4: from /Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler/runtime.rb:24:in `map'
        3: from /Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler/spec_set.rb:136:in `each'
        2: from /Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler/spec_set.rb:136:in `each'
        1: from /Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler/runtime.rb:25:in `block in setup'
/Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/site_ruby/2.7.0/bundler/runtime.rb:309:in `check_for_activated_spec!': You have already activated psych 4.0.1, but your Gemfile requires psych 3.3.2. Prepending `bundle exec` to your command may solve this. (Gem::LoadError)

I am not sure what would be the solution here. Is it possible to execute the git-hooks on a bundle exec context? Is it worth it? (as I imagine there would be performance penalty). 🤔

As a temporary solution (far from ideal), I suppressed the errors by simply uninstall Psych 4.x and YAML 0.1.1, but that's far from the ideal solution I believe.

$ gem uninstall psych:4.0.1
Successfully uninstalled psych-4.0.1

$ git checkout .
Updated 0 paths from the index
/Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/2.7.0/yaml.rb:12: warning: already initialized constant YAML
/Users/Drowze/.asdf/installs/ruby/2.7.3/lib/ruby/gems/2.7.0/gems/yaml-0.1.1/lib/yaml.rb:12: warning: previous definition of YAML was here

$ gem uninstall yaml:0.1.1
Successfully uninstalled yaml-0.1.1

$ git checkout .
Updated 0 paths from the index