Fix renaming a group removes any associated submissions

[ch-iv]

Proposed Changes

Fixes #7215, a bug where renaming a group to an existing group in a different assignment resulted in incorrect repository mapping.

Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
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🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

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[coveralls]

Pull Request Test Coverage Report for Build 11154777481

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 43 of 43 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 91.63%

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Totals
Change from base Build 11097978703:	0.04%
Covered Lines:	40546
Relevant Lines:	43577

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💛 - Coveralls

[david-yz-liu]

@ch-iv great work! I just left one small comment.