Don't modify relative ./ paths in BacktraceCleaner

Previously a path starting with ./ would be replaced to start with /.
IMO this didn't particularly make sense since / reads as though it's
from the root of the filesystem.

This commit removes that filter, preserves ./, and updates the silencer
not to remove lines starting with ./

railties/lib/rails/backtrace_cleaner.rb

@@ -4,7 +4,7 @@
 
 module Rails
   class BacktraceCleaner < ActiveSupport::BacktraceCleaner
-    APP_DIRS_PATTERN = /\A\/?(?:app|config|lib|test|\(\w*\))/
+    APP_DIRS_PATTERN = /\A(?:\.\/)?(?:app|config|lib|test|\(\w*\))/
     RENDER_TEMPLATE_PATTERN = /:in `.*_\w+_{2,3}\d+_\d+'/
 
     def initialize
@@ -20,9 +20,6 @@ def initialize
           line
         end
       end
-      add_filter do |line|
-        line.start_with?("./") ? line.from(1) : line
-      end
       add_silencer { |line| !APP_DIRS_PATTERN.match?(line) }
     end
   end

railties/test/backtrace_cleaner_test.rb

@@ -17,6 +17,15 @@ def setup
     assert_equal 1, result.length
   end
 
+  test "should show relative paths" do
+    backtrace = [ "./test/backtrace_cleaner_test.rb:123",
+                  "/Path/to/rails/activesupport/some_testing_file.rb:42:in `test'",
+                  "bin/rails:4:in `<main>'" ]
+    result = @cleaner.clean(backtrace)
+    assert_equal "./test/backtrace_cleaner_test.rb:123", result[0]
+    assert_equal 1, result.length
+  end
+
   test "can filter for noise" do
     backtrace = [ "(irb):1",
                   "/Path/to/rails/railties/lib/rails/commands/console.rb:77:in `start'",