git: add generic backend implementation for reading git config

[pmrowla]

prerequisites for #278

[codecov-commenter]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
src/scmrepo/git/__init__.py	85.57% <100.00%> (+0.09%)	⬆️
src/scmrepo/git/backend/base.py	81.94% <100.00%> (+0.78%)	⬆️
src/scmrepo/git/config.py	100.00% <100.00%> (ø)
tests/test_git.py	100.00% <100.00%> (ø)
src/scmrepo/git/backend/dulwich/__init__.py	77.35% <71.87%> (-1.36%)	⬇️
src/scmrepo/git/backend/gitpython.py	64.11% <25.00%> (-2.15%)	⬇️
src/scmrepo/git/backend/pygit2/__init__.py	70.54% <53.84%> (-3.18%)	⬇️

... and 4 files with indirect coverage changes

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