Performance improvements in __main__.py

[rogalski]

Do not recompute invariant values in bisect loop.

Fixes #210.

[akaihola]

@rogalski, there seem to be a couple of unfortunate trailing whitespaces. See the build error.

[akaihola]

Could you add a change log entry, and remove the trailing whitespaces caught by the CI build?

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.39%.

Quality metrics	Before	After	Change
Complexity	16.67 🙂	16.82 🙂	0.15 👎
Method Length	101.22 🙂	101.78 🙂	0.56 👎
Working memory	13.48 😞	13.55 😞	0.07 👎
Quality	42.23% 😞	41.84% 😞	-0.39% 👎

Other metrics	Before	After	Change
Lines	372	376	4

Changed files	Quality Before	Quality After	Quality Change
src/darker/main.py	42.23% 😞	41.84% 😞	-0.39% 👎

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/main.py	_reformat_single_file	11 🙂	204 😞	18 ⛔	36.64% 😞	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
src/darker/main.py	print_diff	6 ⭐	90 🙂	10 😞	64.47% 🙂	Extract out complex expressions

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