- Fix https://github.com/ruby/rubygems/pull/9186
- ### Problem

  Running `bundle pristine` in a Gemfile where there is many git gem
  pointing to the same repository will result in a error
  "Another git process seems to be running in this repository".

  ### Context

  This error is a regression since https://github.com/ruby/rubygems/commit/a555fd6ccd17
  where `bundle pristine` now runs in parallel which could lead
  to running simultaneous git operations in the same repository.

  ### Solution

  When Bundler pristine a git gem it does a `git reset --hard` without
  specifying a path.
  This means the whole repository will be reset. In this case, we can
  leverage that by just pristining one gem per unique git sources.
  This is also more efficient.

https://github.com/ruby/rubygems/commit/710ba514a8

https://github.com/rubygems/rubygems/commit/bd2a170330

https://github.com/rubygems/rubygems/commit/6b4cf6713d

https://github.com/rubygems/rubygems/commit/3355c80c90

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

https://github.com/rubygems/rubygems/commit/482077d185

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

s/overriden/overridden/