path-walk: support blob size limit filter

Extend the path-walk API to handle the 'blob:limit=<size>' object
filter natively. This filter omits blobs whose size is equal to or
greater than the given limit, matching the semantics used by the
list-objects-filter machinery.

When revs->filter.choice is LOFC_BLOB_LIMIT, the prepare_filters()
method stores the limit value in info->blob_limit and clears the filter
from revs. If the limit is zero, this degenerates to blob:none (all
blobs excluded), so info->blobs is set to 0 instead.

During walk_path(), blob batches are filtered before being delivered to
the callback: each blob's size is checked via odb_read_object_info(),
and only blobs strictly smaller than the limit are included. Blobs whose
size cannot be determined (e.g. missing in a partial clone) are
conservatively included, matching the existing filter behavior. Empty
batches after filtering are skipped entirely.

The check for inclusion in the path batch looks a little strange at
first glance. We use odb_read_object_info() to read the object's size.
Based on all of the assumptions to this point, this _should_ return
OBJ_BLOB. Since we are focused on the size filter, we use a
short-circuited OR (||) to skip the size check if that method returns a
different object type.

Notice that this inspection of object sizes requires the content to be
present in the repository. The odb_read_object_info() call will download
a missing blob on-demand. This means that the use of the path-walk API
within 'git backfill' would not operate nicely with this filter type.
The intention of that command is to download missing blobs in batches.
Downloading objects one-by-one would go against the point. Update the
validation in 'git backfill' to add its own compatibility check on top
of path_walk_filter_compatible().

Add tests for blob:limit=0 (equivalent to blob:none) and blob:limit=3
(which exercises partial filtering within a batch where some blobs are
kept and others are excluded).

Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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