builtin/gc: move worktree and rerere tasks before object optimizations

In subsequent patches we'll consolidate all tasks that relate to maintenance of the object database and move it into the "files" backend. The relevant code is somewhat scattered though, as several other tasks are interspersed between. Refactor the code so that all object database optimizations are grouped together, which requires us to move worktree pruning and rerere garbage collection around. In theory, rearranging this code can have an effect on the object database optimizations: - Rerere entries really shouldn't impact garbage collection at all, as these entries are not stored in the object database. - The index and HEAD reference of pruned worktrees may reference objects that become unreachable. That being said, the impact should be overall rather negligible. If the user was asking us to prune objects with immediate expiration time then we might now prune objects that were previously still kept alive by the worktree. But besides being a very specific edge case, it's arguably not even the wrong thing to also prune any potentially-unreachable objects immediately. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>