core: Store "pending" objects explicitly, fix pull to use them

core: Store "pending" objects explicitly, fix pull to use them

Before we were creating randomly-named temporary files in repo/tmp
when downloading via pull, but that means if the download process is
interrupted, we have to redownload everything again.

Let's still keep the concept of a "transaction" where files are
stored in the repository as atomically as possible (i.e. we
do a bunch of rename() calls), but now we also have an explicit
"tmp/pending/objects" directory that contains named objects.

This allows us to then skip redownloading things that are pending.