[feature] Do not store the diffs, only the computed hashes and timestamps (or have a limit on the history)

[mdeville]

Is your feature request related to a problem? Please describe.
Sometimes I only need to know if and when the watched resource has been changed. But I don't care about its content.

Describe the solution you'd like
Add an option to store the hashed output of a resource in diff history instead of it's content.

Describe the use-case and give concrete real-world examples
Compagnies that I work with are serving files on an HTTP server.
They are telling me that the file is being refreshed with the latest data every X hour each day.
Those files can be huge, and I don't really need to know what has changed in particular.
I only want to make sure that the file is being refreshed by them.

[dgtlmoon]

Could be solved by a max history length setting for each watch and globally