read workdir from git.config as referenced in man 1 git-config

[Farom]

The setting in .git/config for GIT_WORK_TREE were ignored.
I tried to implement this as git-config says.

[codecov-io]

Codecov Report

Merging #801 into master will decrease coverage by 0.46%.
The diff coverage is 83.33%.

@@ Coverage Diff @@## master #801 +/- ##
==========================================
- Coverage 94.71% 94.25% -0.47% 
==========================================
Files 59 59 Lines 9369 9374 +5 ==========================================
- Hits 8874 8835 -39 - Misses 495 539 +44

Impacted Files	Coverage Δ
git/repo/base.py	95.36% <83.33%> (-0.85%)	⬇️
git/test/test_util.py	90.07% <0%> (-8.52%)	⬇️
git/test/test_base.py	96.42% <0%> (-2.39%)	⬇️
git/objects/submodule/base.py	92.89% <0%> (-1.46%)	⬇️
git/test/test_submodule.py	97.89% <0%> (-1.35%)	⬇️
git/test/lib/helper.py	90.69% <0%> (-1.17%)	⬇️
git/test/test_repo.py	97.24% <0%> (-0.65%)	⬇️
git/cmd.py	83.4% <0%> (-0.44%)	⬇️
git/test/test_docs.py	99.6% <0%> (-0.4%)	⬇️
... and 1 more

---

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[Byron]

Thanks a lot for your contribution! It's much appreciated.
I also added a comment about something puzzling, maybe you can have a look.
Thank you

[Byron]

One change request was made, after which this PR can be merged. See the resolved conversation in git/repo/base.

[Byron]

Thanks a lot for your contribution thus far !
I am closing this PR due to prolonged inactivity. Please feel free to reopen it in case it can proceed.