fetcher: Clean up code to avoid intermediate files

fetcher: Clean up code to avoid intermediate files

I think originally we had the .part/.done separation because we were
trying to support partial downloads of files like repo/config and
repo/refs.

But now that the http server configuration won't give us partial
results, we don't need to support caching those files between runs.

And thus, there's no reason to have the .part/.done and do the dance
with renaming them.

When fetching objects/ and other things that use _with_async, we
continue to use _append_to(), and if the returned range tells us we
have all the bytes, then we hand the full file over to the caller.

Don't attempt to shortcut in the case where the last run told us we
already have the object; the object fetcher code will not make a
request.

While we're here, also clean up use of GError and consistently use the
cancellable from the pending.

https://bugzilla.gnome.org/show_bug.cgi?id=707157