Restore a public API I unintentionally broke in 39857f3.

@DouweM rightly pointed out that I broke this API.

39857f3#commitcomment-2236278

lib/rspec/core/formatters/base_formatter.rb

@@ -117,6 +117,14 @@ def close
           restore_sync_output
         end
 
+        # @api public
+        #
+        # Formats the given backtrace based on configuration and
+        # the metadata of the given example.
+        def format_backtrace(backtrace, example)
+          super(backtrace, example.metadata)
+        end
+
       protected
 
         def configuration

lib/rspec/core/formatters/base_text_formatter.rb

@@ -160,7 +160,7 @@ def format_caller(caller_info)
         end
 
         def dump_backtrace(example)
-          format_backtrace(example.execution_result[:exception].backtrace, example.metadata).each do |backtrace_info|
+          format_backtrace(example.execution_result[:exception].backtrace, example).each do |backtrace_info|
             output.puts cyan("#{long_padding}# #{backtrace_info}")
           end
         end

lib/rspec/core/formatters/html_formatter.rb

@@ -86,7 +86,7 @@ def example_failed(example)
           exception_details = if exception
             { 
               :message => exception.message, 
-              :backtrace => format_backtrace(exception.backtrace, example.metadata).join("\n")
+              :backtrace => format_backtrace(exception.backtrace, example).join("\n")
             }
           else
             false 

spec/rspec/core/formatters/base_formatter_spec.rb

@@ -101,7 +101,7 @@
     end
 
     it "removes lines from rspec and lines that come before the invocation of the at_exit autorun hook" do
-      formatter.format_backtrace(backtrace).should eq(["./my_spec.rb:5"])
+      formatter.format_backtrace(backtrace, stub.as_null_object).should eq(["./my_spec.rb:5"])
     end
   end