#2051 Modifications to self-test to use with gnu diff

[elsapo]

Add argument diff (eg "diff:../path_to_diff/diff.exe")
support.
Revise failure reporting to include exact failing
command line.

(My original version had hardcoded path to gnu diff,
and then I took that out when I revised it to work with
WinMerge; now this version keeps the WinMerge support,
but adds back in support for use with gnu diff, via an
option giving the path to the diff program.)

[elsapo]

7-Zip file (4Kb) of new version

[Kimmo Varis]

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I'd rather start creating unit tests...