Support monorepo/root in subfolder

[filmhubhq]

Say you have a monorepo with

/ (repo root)
├── rails
└── web

This breaks coverage reports (for example for CodeBuild) because of missing rails/ and web/ prefixes in all paths.

We have found a workaround with

# rails/test/support/coverage.rb, required in test_helper.rb:
SimpleCov.start 'rails' do
  # test file paths need to include `rails/...` for coverage report
  root File.expand_path('../../..', __dir__)
  # .. but move the coverage reports back into `rails/test/coverage`
  coverage_dir 'rails/coverage'
end

but that's rather cumbersome.

The issue is that this is not configurable:

simplecov-lcov/lib/simplecov-lcov.rb

Line 85 in 61b771f

filename = file.filename.gsub("#{SimpleCov.root}/", './')

Would you be open for a PR? If so, any preferred config API? Maybe simply like this?

SimpleCov::Formatter::LcovFormatter.config.project_root = `rails/`

Or should this be a SimpleCov PR?