RF: No GitPython use in publish

[mih]

Sits on top of #3834 because it needs get_branch_commits() to return string hex SHAs.

[codecov]

Codecov Report

Merging #4171 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4171      +/-   ##
==========================================
+ Coverage   89.13%   89.17%   +0.04%     
==========================================
  Files         275      275              
  Lines       35847    35825      -22     
==========================================
- Hits        31953    31948       -5     
+ Misses       3894     3877      -17     

Impacted Files	Coverage Δ
datalad/support/tests/test_annexrepo.py	95.57% <0.00%> (+0.32%)	⬆️
datalad/downloaders/base.py	75.55% <0.00%> (+0.37%)	⬆️
datalad/downloaders/http.py	74.10% <0.00%> (+0.39%)	⬆️
datalad/downloaders/tests/test_http.py	61.80% <0.00%> (+1.21%)	⬆️
datalad/support/tests/utils.py	96.15% <0.00%> (+11.53%)	⬆️
datalad/support/tests/test_cookies.py	100.00% <0.00%> (+14.28%)	⬆️

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[kyleam]

Sits on top of #3834 because it needs get_branch_commits() to return string hex SHAs.

I already commented that the changes from gh-3834 look fine to me. The new changes here do too.