refs/files: use heuristic to decide whether to repack with `--auto`

The --auto flag for git-pack-refs(1) allows the ref backend to decide whether or not a repack is in order. This switch has been introduced mostly with the "reftable" backend in mind, which already knows to auto-compact its tables during normal operations. When the flag is set, then it will use the same auto-compaction mechanism and thus end up doing nothing in most cases.

The "files" backend does not have any such heuristic yet and instead packs any loose references unconditionally. So we rewrite the complete "packed-refs" file even if there's only a single loose reference to be packed. Even worse, git pack-refs --auto is also executed via our auto maintenance, so we end up repacking references over and over again.

Introduce a heuristic that decides whether or not it is worth to pack loose references. The important factors to decide here are the number of loose references in comparison to the overall size of the "packed-refs" file. The bigger the "packed-refs" file, the longer it takes to rewrite it and thus we scale up the limit of allowed loose references before we repack.

As is the nature of heuristics, this mechansim isn't obviously "correct", but should rather be seen as a tradeoff between how much resources we spend packing refs and how inefficient the ref store becomes. For all I can say, we have successfully been using the exact same heuristic in Gitaly for several years by now.

Closes Auto-maintenance always packs loose references (#379 - closed).