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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