fix(remote) Fix detection of GitLab merge request sha

[hackenbergstefan]

Description

GitLabMergeRequest::merge_commit() now returns the commit's sha instead of None if the Merge Request was not merged using a merge commit but with fast-forward.

Motivation and Context

In the generation of the changelog commits were not linked to their GitLab MR if the commit was merged using fast-forward.
This change allows to identify the corresponding GitLab MR independently if the MR was merged with a merge commit or with fast-forward.

How Has This Been Tested?

Existing tests are sufficient and still pass.

Screenshots / Logs (if applicable)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly. [imho not necessary]
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[welcome]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.35%. Comparing base (c449229) to head (ad03aea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #968   +/-   ##
=======================================
  Coverage   43.35%   43.35%           
=======================================
  Files          21       21           
  Lines        1712     1712           
=======================================
  Hits          742      742           
  Misses        970      970           

Flag	Coverage Δ
unit-tests	43.35% <100.00%> (ø)

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