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

imsg_clear frees and closes resources allocated as part of enqueueing imsgs so it's a no-op after reads. discussed with and ok stsp@

tested by thomas, naddy, and myself

millert@ suggests that this check is not needed, and that ideally we should be using close-on-exec instead. I will look into this, but in the meantime this change will help -portable: https://bugs.gentoo.org/828003

Feature requested by naddy. ok naddy, who also suggested some tweaks that will arrive shortly

Known to work against git-daemon and github Git server implementations. Tests by abieber, naddy, jrick, and myself. Man page additions reviewed by Lucas.

This saves an iteration over the entire h->offsets array when opening a pack index which should not contain large offsets in the first place. ok millert@

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.