[FEATURE] Enhance diff output for files with no differences

[OdatNurd]

Summary

Currently, when a diff indicates that there are no changes between an override and its source file, the only text displayed is some placeholder text to tell you that there are no differences.

For some use cases/some users, it would be useful if such a diff still contained the diff context header that indicates the two files that participated in the diff and their relative time stamps.

Since this is not the usual behaviour anyone expects from a diff, if implemented this would be a configurable item that defaults to being disabled so that users that want/need this information can enable it if desired.

Such a setting could be allowed to control if this happens for only single file diffs or also for bulk diffs as well. In the case of a bulk diff the utility of the header would be to tell you the time stamps of the two files, since the location of the files can be determined by looking at the package header.

[OdatNurd]

This is currently implemented purely as an on/off toggle for empty diff headers (i.e. everywhere or nowhere). If any future interest is garnered for this being only partially enabled (say only in a bulk diff or only for an expired override) that could be added as a new issue.