The Git access checks support checking the size of pushed objects. If we
have a quarantine environment (which is always the case when access
checks are executed via `git receive-pack`'s hooks), then we simply
derive the size from the quarantined objects. Otherwise, we use
`get_new_blobs`, which does a `git rev-list $REVISION --not --all`.

Using git-rev-list(1) with `--not --all` can be a very expensive
operation depending on the repository's shape. Especially when it's a
biggish monorepo with lots of references, this query can easily take
tens of seconds. Given that we call the RPC once per change, this thus
roughly scales `O(len(changes) * len(existing_refs))`.

To improve this situation, Gitaly has implemented a new `list_blobs()`
RPC which takes a set of revisions. Like this, we can batch all calls
into a single one and thus avoid some of the overhead if there are
multiple changes at once. The new code which does that is currently
implemented behind a feature flag.

Changelog: performance