Add a test for the `parse_line_with_relative_date` function in test_parsing.py

[todo[bot]]

https://github.com/frou/st3-gitblame/blob/ba973acb1f75ef50849bdd19825e9476e59c4a35/src/base.py#L66-L69

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This issue was generated by todo based on a todo comment in ba973acb1f75ef50849bdd19825e9476e59c4a35. It's been assigned to @frou because they committed the code.