Remove dependency on GitPython

Replace usage of random low-level parts of GitPython with a simple
convenience functionality similar to that offered by git.cmd. For
example:

 git.commit(file="-", amend=True, _input=input)

Include special options _input/_quiet/_interactive to allow removing
several cases where subprocess.Popen() was used directly and improve
output on git command failure.

Also, use commit.subject rather than commit.message to be consistent
with standard Git terminology.

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