codeintel: fix nil pointer in retention policy match overview for visible to tip-of-default-branch uploads
Created by: Strum355
Addresses bug when a non tip-of-default-branch upload was retained by being visible to a tip-of-default-branch commit, causing nil pointer as an incorrect assumption was made (see deleted comment)
Also updated UI to limit number of visible commits listed.
Test plan
Added unit tests and manually tested in UI