Thanks Dan, Dennis.
I am a bit in a hurry and ended up hacking GitClass:getFile() so that it would generate the correct git path for a file so that I can do diffs against the latest head revision:
// cmdLine := maybe_quote_filename(exeStr)" --no-pager show "versionStr':'maybe_quote_filename('./'_strip_filename(gitFilename,'P'));
cmdLine := maybe_quote_filename(exeStr)" --no-pager show "versionStr':'maybe_quote_filename(gitFilename);
This probably going to break something else but it works for what I need to do. I need to to a bit more research before I can file a defect. It is such a drastic failure that I should have seen it earlier, so it may have something to do with the ancient git 1.7.1 version in my distro.