Always mark the string returned by File.realpath as tainted

This string can include elements that were not in either string
passed to File.realpath, even if one of the strings is an
absolute path, due to symlinks:

```ruby
Dir.mkdir('b') unless File.directory?('b')
File.write('b/a', '') unless File.file?('b/a')
File.symlink('b', 'c') unless File.symlink?('c')
path = File.realpath('c/a'.untaint, Dir.pwd.untaint)
path # "/home/testr/ruby/b/a"
path.tainted? # should be true, as 'b' comes from file system
```

[Bug #15803]

2 files changed, 2 insertions, 2 deletions

diff --git a/file.c b/file.c
index 7ab2f2a026..a04fe538e5 100644
--- a/file.c
+++ b/file.c
@@ -4270,7 +4270,7 @@ rb_check_realpath_internal(VALUE basedir, VALUE path, enum rb_realpath_mode mode
 	}
     }
 
-    OBJ_INFECT(resolved, unresolved_path);
+    rb_obj_taint(resolved);
     RB_GC_GUARD(unresolved_path);
     RB_GC_GUARD(curdir);
     return resolved;
diff --git a/test/ruby/test_file.rb b/test/ruby/test_file.rb
index 5e9574cf32..36c154d36c 100644
--- a/test/ruby/test_file.rb
+++ b/test/ruby/test_file.rb
@@ -298,7 +298,7 @@ class TestFile < Test::Unit::TestCase
       assert_predicate(File.realpath(base, dir), :tainted?)
       base.untaint
       dir.untaint
-      assert_not_predicate(File.realpath(base, dir), :tainted?)
+      assert_predicate(File.realpath(base, dir), :tainted?)
       assert_predicate(Dir.chdir(dir) {File.realpath(base)}, :tainted?)
     }
   end