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

The failure is in the git hook script that calls Bundler.setup after loading .overcommit.yml, so start with the hook template Overcommit installs and how it sets BUNDLE_GEMFILE. Reproduce by pinning psych 3.x in a Gemfile while psych 4.x is also installed, then run the hook. Done would mean the hook loads the Gemfile's psych without the already-activated conflict; the issue has no comments or linked fix, so the approach is open and needs maintainer input.

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