This prepares for callers of got_repo_open() that cannot afford to open files in /tmp, such as gotwebd. In a follow-up change, we could ask such callers to pass in the required amount of open temporary files. One consequence is that got_repo_open() now requires the "cpath" pledge promise. Add the "cpath" promise to affected callers and remove it once the repository has been opened. ok tracey

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@

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

got-read-pack must rewind its files temporary files before reusing them. Problem reported by naddy ok tracey naddy

ok thomas_adam

suggested by millert ok thomas_adam

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@