Move to `SubprocessCommand`, document

[tony]

Fixes #340

[codecov[bot]]

Codecov Report

Merging #341 (5e5cbc4) into master (eb14d10) will increase coverage by 0.01%. The diff coverage is n/a.

:exclamation: Current head 5e5cbc4 differs from pull request most recent head 9a8e0f1. Consider uploading reports for the commit 9a8e0f1 to get more accurate results

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   70.33%   70.35%   +0.01%     
==========================================
  Files          21       21              
  Lines        1888     1882       -6     
==========================================
- Hits         1328     1324       -4     
+ Misses        560      558       -2     

Impacted Files	Coverage Δ
libvcs/projects/git.py	76.53% <0.00%> (+0.12%)	:arrow_up:
docs/conf.py	62.76% <0.00%> (+0.40%)	:arrow_up:

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