differential test coverage when working on a patch

When testing my own patches or review other's patches, I want to know if
the new code has been tested, however our current 'make coverage-html'
shows all the codes rather than the 'new code', so is there a good way
to get the answer for the above question?

I searched lcov at [1]https://github.com/linux-test-project/lcov/blob/master/README and the options like '--diff-file' or
'--select-script' looks very promising, but all of them needs some time
to try it out and then automate it. so I'd like to ask first..

[1]: https://github.com/linux-test-project/lcov/blob/master/README

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Best Regards
Andy Fan