add some comments and additional test case

lib/datadog/ci/git/local_repository.rb

@@ -30,21 +30,26 @@ def self.root
           @root = git_root || Dir.pwd
         end
 
+        # ATTENTION: this function is running in a hot path
+        # and should be optimized for performance
         def self.relative_to_root(path)
           return "" if path.nil?
 
           root_path = root
           return path if root_path.nil?
 
           if File.absolute_path?(path)
+            # prefix_index is where the root path ends in the given path
             prefix_index = root_path.size
 
-            # impossible case
+            # impossible case - absolute paths are returned from code coverage tool that always checks
+            # that root is a prefix of the path
             return "" if path.size < prefix_index
 
             prefix_index += 1 if path[prefix_index] == File::SEPARATOR
             res = path[prefix_index..]
           else
+            # prefix_to_root is a difference between the root path and the given path
             if @prefix_to_root == ""
               return path
             elsif @prefix_to_root

spec/datadog/ci/test_optimisation/coverage/event_spec.rb

@@ -104,6 +104,41 @@
       end
     end
 
+    context "when file paths are relative" do
+      before do
+        Datadog::CI::Git::LocalRepository.remove_instance_variable(:@prefix_to_root)
+
+        allow(Datadog::CI::Git::LocalRepository).to receive(:root).and_return(
+          Dir.pwd.gsub("/datadog-ci-rb", "")
+        )
+      end
+
+      after do
+        Datadog::CI::Git::LocalRepository.remove_instance_variable(:@prefix_to_root)
+      end
+
+      let(:coverage) do
+        {
+          "project/file.rb" => true,
+          "project/file2.rb" => true
+        }
+      end
+
+      it "converts all file paths to relative to the git root" do
+        expect(msgpack_json).to eq(
+          {
+            "test_session_id" => 3,
+            "test_suite_id" => 2,
+            "span_id" => 1,
+            "files" => [
+              {"filename" => "datadog-ci-rb/project/file.rb"},
+              {"filename" => "datadog-ci-rb/project/file2.rb"}
+            ]
+          }
+        )
+      end
+    end
+
     context "coverage in lines format" do
       let(:coverage) { {"file.rb" => {1 => true, 2 => true, 3 => true}} }