Improve performance for Combine.combine

Create less intermediate objects.

Author: splattael

lib/simplecov/combine/lines_combiner.rb

@@ -10,11 +10,9 @@ module LinesCombiner
     module_function
 
       def combine(coverage_a, coverage_b)
-        coverage_a
-          .zip(coverage_b)
-          .map do |coverage_a_val, coverage_b_val|
-            merge_line_coverage(coverage_a_val, coverage_b_val)
-          end
+        coverage_a.map.with_index do |coverage_a_val, index|
+          merge_line_coverage(coverage_a_val, coverage_b[index])
+        end
       end
 
       # Return depends on coverage in a specific line