This structure is used to provide callers information about the progress of indexing a packfile, either directly or part of a fetch or clone that downloads a packfile.
number of objects in the packfile being indexed
received objects that have been hashed
received_objects: objects which have been downloaded
locally-available objects that have been injected in order to fix a thin pack
number of deltas in the packfile being indexed
received deltas that have been indexed
size of the packfile received up to now