Update How to run Infection only for changed files topic

[Slamdunk]

Hi, 4 years ago How to run Infection only for changed files was introduced in the How-to section, which is using the (still valid) CHANGED_FILES=$(git diff ...) method.

But now we have --git-diff-filter #183 and --git-diff-lines #228, so I think it is worth updating the main How-to guide to use the newer options, if I'm not missing anything.

[maks-rafalko]

Thanks for this suggestion, @Slamdunk

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