I am looking for a lightweight way to notate changes made to a file. The obvious answer to this is a version control system. But this is more robust than necessary for what I would like to do. All I want is the functionality to compare an edited file to its original in a lightweight manner.
Ways of marking changes being made to a file
version control;file systems
You might want to check the set of tools diff, cmp, sdiff, diff3, etc. These will provide you with easy to parse text output, making them usable from another program. diff in particular would probably do what you want.