Backport #13136
it is possible to have an ambiguous line here.
if they needed to be and if one was quoted then both would be.
Both of these were wrong.
I have now discovered `--src-prefix` and `--dst-prefix` which
means that we can set this in such a way to force the git diff
to always be unambiguous.
Therefore this PR rollsback most of the changes in #12771 and
uses these options to fix this.
Signed-off-by: Andrew Thornton <art27@cantab.net>
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