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.

ok millert, naddy

ok millert, naddy