[SDTEST-126] add different fallbacks for unshallowing remotes

[anmarchenko]

What does this PR do?
Our current git unshallowing command is optimised, but it might fail under a different circumstances.

For example git rev-parse HEAD fails if the local HEAD is a commit that has not been pushed to the remote.

We will add the following fallbacks:

• $(git rev-parse --abbrev-ref --symbolic-full-name @{upstream}) - we try this first, but it might fail too if the CI is working on a detached HEAD or maybe branch tracking hasn’t been set up
• provide no remote as a last resort

Motivation
Improve git unshallowing resilience

How to test the change?
Unit test is provided

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.91%. Comparing base (d56ec05) to head (c5ac583).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
+ Coverage   98.89%   98.91%   +0.02%     
==========================================
  Files         261      261              
  Lines       12291    12310      +19     
  Branches      557      558       +1     
==========================================
+ Hits        12155    12177      +22     
+ Misses        136      133       -3     

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