Add a merge_base_finder_test unit test that simulates workloads that perform many merges

[talSofer]

Add a unit test that could have caught the diff bug that was fixed in https://github.com/treeverse/lakeFS/pull/2968 https://github.com/treeverse/lakeFS/blob/master/pkg/graveler/ref/merge_base_finder_test.go

[talSofer]

FYI @itai-david you may have a scenario in mind, after working on a large-scale case.

[github-actions[bot]]

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