[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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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