[CIVIS-2938] Git tree upload - git command line integration

[anmarchenko]

What does this PR do?

• Consolidates all calls to external git commands in new module Datadog::CI::Git::LocalRepository
• Makes exec_git_command inaccessible from outside of this module to make shell injections less likely by reviewing that all calls to this method are done with static parameters
• Adds git_commits, git_commits_rev_list, git_generate_packfiles methods to support git tree uploading functionality

How to test the change?
Unit tests are provided

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.61111% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 99.03%. Comparing base (9716194) to head (f9a6d5c).

Files	Patch %	Lines
lib/datadog/ci/git/local_repository.rb	95.83%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              1.0     #151      +/-   ##
==========================================
+ Coverage   99.02%   99.03%   +0.01%     
==========================================
  Files         179      182       +3     
  Lines        8090     8229     +139     
  Branches      339      341       +2     
==========================================
+ Hits         8011     8150     +139     
  Misses         79       79              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.