[BZ#777675] Make reduction of comparison criteria to size and timestamp a comparison option instead of an application option

Submitted by Kalle Richter

Description

The option to limit the comparison to the file size and the modification timestamp should be set at every comparison because

one might forget that it has been set and misinterpret the comparison result

one might want to run comparisons in parallel (since meld allow that) and use content comparison for the one and size/timestamp comparison for the other

The initial value might be configurable (e.g. always content-based, always size/timestamp or remember last) or not.