[CIVIS-2863] Git repository unshallowing logic

[anmarchenko]

What does this PR do?
Adds the following git unshallowing logic to this library:

Also implements unshallowing optimization: if all commits from the shallow repo are present in the backend, we skip the git upload alltogether.

How to test the change?
Unit tests are provided

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.42857% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 99.00%. Comparing base (86d6c60) to head (2bcb5f0).

Files	Patch %	Lines
lib/datadog/ci/git/local_repository.rb	55.55%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              1.0     #155      +/-   ##
==========================================
- Coverage   99.04%   99.00%   -0.04%     
==========================================
  Files         190      190              
  Lines        8595     8665      +70     
  Branches      369      371       +2     
==========================================
+ Hits         8513     8579      +66     
- Misses         82       86       +4     

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