[CIVIS-2938] Upload packfiles API client

[anmarchenko]

What does this PR do?
Adds Git::UploadPackfile class that is able to upload a single git packfile to Datadog's git API.

Makes minor changes to git_commits_rev_list and git_generate_packfiles methods to make them safer and prevent shell injections.

How to test the change?
Unit tests are provided, otherwise this functionality is not used yet

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.87640% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.02%. Comparing base (e238a74) to head (40cd927).

Files with missing lines	Patch %	Lines
spec/datadog/ci/git/upload_packfile_spec.rb	97.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              1.0     #153      +/-   ##
==========================================
- Coverage   99.05%   99.02%   -0.03%     
==========================================
  Files         184      186       +2     
  Lines        8341     8421      +80     
  Branches      346      352       +6     
==========================================
+ Hits         8262     8339      +77     
- Misses         79       82       +3     

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