Occasionally, you may need to do some clean up of a repository, usually to make it more compact. Such clean ups are a very important step after migrating a repository from another version control system.

Modern Git (or rather all but ancient Git) from time to time runs the git gc --auto command in each repository. This command checks whether there are too many loose objects (objects stored as separate files, one file per object, rather than stored together in a packfile; objects are almost always created as loose), and if these conditions are met, then it would launch the garbage collection operation. The garbage collection means to gather up all the loose objects and place them in packfiles, and to consolidate many small packfiles into one large packfile. Additionally, it packs references into the packed-refs file. Objects that are unreachable even via reflog, and are safely old, do not get picked in the repack. Git would then delete loose objects and packfiles that...