Hide expected Git errors from stderr

[akaihola]

Fixes #85.

With e.g. darker --revision=master..., newly created files would cause this Git error to appear on stderr:

fatal: Path 'newfile.py' exists on disk, but not in 'master'.

We already caught that situation and interpreted it properly as caused by a newly created file. In addition, the Git error messages are now silenced, but other Git errors are still logged with the ERROR loglevel.

Unfortunately I couldn't find a way to create a failing test, i.e. one that would be able to catch Git's stderr.

[akaihola]

@casio would you be interested to review this PR? I invited you as collaborator in this project in case you would.

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 1.97%.

Quality metrics	Before	After	Change
Complexity	2.15 ⭐	2.31 ⭐	0.16 👎
Method Length	78.31 🙂	82.07 🙂	3.76 👎
Working memory	6.70 🙂	7.36 🙂	0.66 👎
Quality	75.59% ⭐	73.62% 🙂	-1.97% 👎

Other metrics	Before	After	Change
Lines	668	738	70

Changed files	Quality Before	Quality After	Quality Change
src/darker/git.py	75.65% ⭐	73.68% 🙂	-1.97% 👎
src/darker/tests/test_git.py	75.55% ⭐	73.59% 🙂	-1.96% 👎

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

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
src/darker/tests/test_git.py	test_git_check_output_lines_stderr_and_log	0 ⭐	153 😞	15 😞	54.73% 🙂	Try splitting into smaller methods. Extract out complex expressions
src/darker/tests/test_git.py	test_git_get_modified_files	4 ⭐	215 ⛔	8 🙂	57.62% 🙂	Try splitting into smaller methods
src/darker/git.py	git_get_modified_files	6 ⭐	134 😞	9 🙂	60.82% 🙂	Try splitting into smaller methods
src/darker/tests/test_git.py	test_git_check_output_lines	2 ⭐	154 😞	9 🙂	62.56% 🙂	Try splitting into smaller methods
src/darker/tests/test_git.py	test_git_get_content_at_revision	2 ⭐	152 😞	9 🙂	62.77% 🙂	Try splitting into smaller methods

Legend and Explanation

The emojis denote the absolute quality of the code:

• ⭐ excellent
• 🙂 good
• 😞 poor
• ⛔ very poor

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

---

Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Help us improve this quality report!

[flying-sheep]

Hi, you forgot to set a language for running git.

If you want to parse the localized output of some tool, you need to select a localization.

[akaihola]

@flying-sheep, that's an excellent catch! Could you please open a separate issue about that?