This significantly speeds up the deltification step of packing by avoiding imsg traffic. gotadmin no longer requests individual raw deltas from got-read-pack to check whether it can reuse them. Instead, got-read-pack obtains a list of objects we want to pack, and hands back the list of all deltas in its pack file which can be reused. Messages are now batched such that imsg buffers are filled as much as possible. Another advantage is that deltas we are not going to reuse will no longer be written to the delta cache file, saving disk space. Before this patch, any raw delta candidate was written to the delta cache file by got-read-pack, and the decision whether to reuse the delta happened afterwards in the gotadmin process. Code for reading individual raw deltas is now unused and could be removed at some point. ok op@

This reduces memory and disk space consumption during packing. with tweaks + memleak on error fix from op@ ok op@

Saves us from doing a malloc/free call for every item on the list. ok op@

Instead of internally opening and closing the same commit object over and over again, require callers to pass an open commit object in. Avoids an inherent dependency on the commit object cache for reasonable performance. ok op@

tested by thomas, naddy, and myself

This speeds up the creation of pack files by about 30%.

suggested by millert ok thomas_adam

Need for filtering by path sugggested by kn@ ok naddy@

The singly-linked tail queue macros were added to OpenBSD 6.9 and are more widely available on other systems. ok stsp

This way, the size of a raw object is the same regardless of whether the object was found in a loose object file or in a pack file.

This will be required by a future 'gotadmin pack' command.