Common ancestor fix

[akaihola]

Bugfix: <commit>... now compares always correctly to the latest common ancestor

This merge request used to be even larger, but I've managed to trim it down. I hope someone is willing to review it in one go.

Note: #107 will fix the build failure.

[akaihola]

100% test coverage for modified lines

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.89%.

Quality metrics	Before	After	Change
Complexity	5.17 ⭐	5.22 ⭐	0.05 👎
Method Length	76.27 🙂	79.55 🙂	3.28 👎
Working memory	14.52 😞	14.65 😞	0.13 👎
Quality	60.15% 🙂	59.26% 🙂	-0.89% 👎

Other metrics	Before	After	Change
Lines	1194	1377	183

Changed files	Quality Before	Quality After	Quality Change
src/darker/main.py	41.28% 😞	41.08% 😞	-0.20% 👎
src/darker/git.py	78.72% ⭐	72.98% 🙂	-5.74% 👎
src/darker/linting.py	66.04% 🙂	64.63% 🙂	-1.41% 👎
src/darker/tests/test_command_line.py	59.12% 🙂	58.94% 🙂	-0.18% 👎
src/darker/tests/test_git.py	65.64% 🙂	63.99% 🙂	-1.65% 👎
src/darker/tests/test_linting.py	54.12% 🙂	53.97% 🙂	-0.15% 👎
src/darker/tests/test_main.py	69.09% 🙂	68.52% 🙂	-0.57% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
src/darker/main.py	format_edited_parts	22 😞	210 ⛔	18 ⛔	27.55% 😞	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_command_line.py	test_parse_command_line	4 ⭐	469 ⛔	60 ⛔	30.03% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/main.py	main	16 🙂	216 ⛔	12 😞	38.76% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_command_line.py	test_black_options	0 ⭐	194 😞	20 ⛔	46.59% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_main.py	test_main	0 ⭐	176 😞	23 ⛔	46.60% 😞	Try splitting into smaller methods. Extract out complex expressions

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• 😞 poor
• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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