Allow pre-commit -o HEAD

[akaihola]

With this patch, Darker will modify files when called by pre-commit -o HEAD, and emit a warning about this being an experimental feature.

With any other values than HEAD for pre-commit -o, Darker will still refuse to modify files.

Fixes #180. See the issue for detailed discussion.

[sourcery-ai]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 1.65%.

Quality metrics	Before	After	Change
Complexity	8.90 🙂	9.01 🙂	0.11 👎
Method Length	105.38 🙂	103.46 🙂	-1.92 👍
Working memory	12.70 😞	12.18 😞	-0.52 👍
Quality	52.57% 🙂	54.22% 🙂	1.65% 👍

Other metrics	Before	After	Change
Lines	886	934	48

Changed files	Quality Before	Quality After	Quality Change
src/darker/main.py	43.40% 😞	42.23% 😞	-1.17% 👎
src/darker/tests/test_main.py	63.27% 🙂	66.30% 🙂	3.03% 👍

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	main	29 😞	365 ⛔	15 😞	20.34% ⛔	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_main.py	test_main	6 ⭐	434 ⛔	24 ⛔	30.58% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/main.py	_reformat_single_file	10 🙂	199 😞	18 ⛔	37.92% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_main.py	test_format_edited_parts	0 ⭐	241 ⛔	14 😞	49.14% 😞	Try splitting into smaller methods. Extract out complex expressions
src/darker/main.py	format_edited_parts	10 🙂	84 🙂	11 😞	59.66% 🙂	Extract out complex expressions

Legend and Explanation

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• 🙂 good
• 😞 poor
• ⛔ very poor

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

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