https://github.com/ruby/rubygems/commit/33c7a9a565

https://github.com/rubygems/rubygems/commit/573ffad3ea

It only affected the `--path` flag which is actually getting removed, so
I don't think it makes sense to make such change. The current behavior
is reasonable and I tried to codify it with a few more specs.

https://github.com/rubygems/rubygems/commit/6f520eb146

Since now every functionality that changes in Bundler 4 is under a
setting, we can enable that setting to test the new functionality,
without having to run our full CI twice.

This can actually be seen as increasing coverage, because Bundler 4
functionality will now be tested on Windows, MacOS, or any other
environment where previously "Bundler 4 mode" was not running.

https://github.com/rubygems/rubygems/commit/1cb3e009fc

https://github.com/rubygems/rubygems/commit/28b6a7cf5e

Because it does not swallow errors if it fails to remove the given
folders, making issues easier to debug.

https://github.com/rubygems/rubygems/commit/0db12d7afc

https://github.com/rubygems/rubygems/commit/bb66253f2c

When testing under the ruby/ruby setup, mkmf.rb needs to the `$extout`
global variable set properly.

This is because, in this particular case, the `ruby.h` header needed to
compile extensions is constructed from
`$(extout)/include($arch)/ruby/config.h` but `$extout` is not set by
default.

I tried to fix this in mkmf.rb itself but I couldn't figure it. But
setting it externally to workaround the issue fixes the specs, so I'll
start with that. Also setting it externally causes issues when running
specs upstream against Ruby 2.3 (I guess because of some difference with
Ruby 2.3 mkmf.rb implementation). So I'm avoiding doing it on Ruby 2.3 to
woraround that.

https://github.com/rubygems/rubygems/commit/d782984585