Resolve test incompatibility with git v2.21.0

[jeking3]

This fixes#869

[codecov-io]

Codecov Report

Merging #870 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #870 +/- ##
==========================================
+ Coverage 94.66% 94.79% +0.12% 
==========================================
Files 59 59 Lines 9393 9607 +214 ==========================================
+ Hits 8892 9107 +215 + Misses 501 500 -1

Impacted Files	Coverage Δ
git/test/test_remote.py	97.84% <100%> (+0.29%)	⬆️
git/test/test_docs.py	100% <0%> (ø)	⬆️
git/objects/blob.py	100% <0%> (ø)	⬆️
git/refs/remote.py	100% <0%> (ø)	⬆️
git/test/test_diff.py	100% <0%> (ø)	⬆️
git/test/performance/test_streams.py	100% <0%> (ø)	⬆️
git/test/test_commit.py	100% <0%> (ø)	⬆️
git/test/test_tree.py	100% <0%> (ø)	⬆️
git/test/test_exc.py	100% <0%> (ø)	⬆️
git/test/test_refs.py	99.18% <0%> (ø)	⬆️
... and 27 more

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