Reuse SimpleCov.grouped from Result#groups

`Result` carried a private `apply_groups` that was line-for-line
identical to `SimpleCov.grouped` except for the source of the file
set and group config. Add an optional `groups:` keyword to
`SimpleCov.grouped` (defaulted to `SimpleCov.groups`) so `Result#groups`
can delegate, and drop the duplicate.

Author: sferik

lib/simplecov/result.rb

@@ -74,9 +74,9 @@ def coverage_for(path)
       source_file_for(path)&.coverage_statistics
     end
 
-    # Returns a Hash of groups for this result. Define groups using SimpleCov.add_group 'Models', 'app/models'
+    # Returns a Hash of groups for this result. Define groups using SimpleCov.group 'Models', 'app/models'
     def groups
-      @groups ||= apply_groups
+      @groups ||= SimpleCov.grouped(files, groups: @groups_config)
     end
 
     # Applies the configured SimpleCov.formatter on this result. Returns
@@ -154,22 +154,5 @@ def apply_cover_filters!(cover_filters)
         @files.select { |source_file| cover_filters.any? { |filter| filter.matches?(source_file) } }
       )
     end
-
-    # Build the per-group FileLists from `@groups_config`, plus the
-    # implicit "Ungrouped" bucket for files that no group filter
-    # matched.
-    def apply_groups
-      return {} if @groups_config.empty?
-
-      grouped = @groups_config.transform_values do |filter|
-        SimpleCov::FileList.new(files.select { |source_file| filter.matches?(source_file) })
-      end
-
-      in_group  = grouped.values.flat_map(&:to_a)
-      ungrouped = files.reject { |source_file| in_group.include?(source_file) }
-      grouped["Ungrouped"] = SimpleCov::FileList.new(ungrouped) if ungrouped.any?
-
-      grouped
-    end
   end
 end

lib/simplecov/result_processing.rb

@@ -59,8 +59,12 @@ def filtered(files)
       SimpleCov::FileList.new result
     end
 
-    # Applies the configured groups to the given array of SimpleCov::SourceFile items
-    def grouped(files)
+    # Bin the given source files by group filter. `groups:` defaults to
+    # `SimpleCov.groups`; pass a Hash explicitly to bin against a
+    # different group config (e.g., the snapshot a Result captured at
+    # construction). Files matched by no group fall into the implicit
+    # "Ungrouped" bucket.
+    def grouped(files, groups: self.groups)
       return {} if groups.empty?
 
       grouped = groups.transform_values do |filter|