Fix updating of annotation overall comment

This fixes #1431. The instance variable `@result` is always the new
result of the remark, not the original old result. The caller of the
template passes the old result correctly as a local variable `result`,
but the template is not using it.

Tested:
- local server
- rake test:unit
- rake test:functional
Add functional test to cover comment edit forms
Merge pull request #1438 from zhangsu/fix_annotation_overall_comment

Fixes #1431.