Re: [PATCH 2/9] builtin/cat-file: wire up an option to filter objects

> In batch mode, git-cat-file(1) enumerates all objects and prints them
> by iterating through both loose and packed objects. This works without
> considering their reachability at all, and consequently most options to
> filter objects as they exist in e.g. git-rev-list(1) are not applicable.
> In some situations it may still be useful though to filter objects based
> on properties that are inherent to them. This includes the object size
> as well as its type.
>
> Such a filter already exists in git-rev-list(1) with the `--filter=`
> command line option. While this option supports a couple of filters that
> are not applicable to our usecase, some of them are quite a neat fit.
>
> Wire up the filter as an option for git-cat-file(1). This allows us to
> reuse the same syntax as in git-rev-list(1) so that we don't have to
> reinvent the wheel. For now, we die when any of the filter options has
> been passed by the user, but they will be wired up in subsequent
> commits.
>
> Note that we don't use the same `--filter=` name fo the option as we use

> in git-rev-list(1). We already have `--filters`, and having both
> `--filter=` and `--filters` would be quite confusing. Instead, the new
> option is called `--objects-filter`.
>
> Further note that the filters that we are about to introduce don't
> significantly speed up the runtime of git-cat-file(1). While we can skip
> emitting a lot of objects in case they are uninteresting to us, the
> majority of time is spent reading the packfile, which is bottlenecked by
> I/O and not the processor. This will change though once we start to make
> use of bitmaps, which will allow us to skip reading the whole packfile.
>