Fix #6104: Use sub-process in pre_push_hook

[lilithxxx]

Explanation

Fixes #6104
Pre_push_hook now uses sub-process instead of gitpython to compare against remote

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
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[codecov-io]

Codecov Report

Merging #6114 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6114   +/-   ##
========================================
  Coverage    45.32%   45.32%           
========================================
  Files          525      525           
  Lines        30861    30861           
  Branches      4614     4614           
========================================
  Hits         13989    13989           
  Misses       16872    16872

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[seanlip]

Thanks for removing the third-party libs!