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

[todo]

st3-gitblame/src/base.py

Lines 66 to 69 in ba973ac

	# @todo Add a test for the `parse_line_with_relative_date` function in test_parsing.py
	@classmethod
	def parse_line_with_relative_date(cls, line):
	"""

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This issue was generated by todo based on a todo comment in ba973ac. It's been assigned to @frou because they committed the code.