path-walk: support blobless filter

path-walk: support blobless filter

The 'git pack-objects' command can opt-in to using the path-walk API for
scanning the objects. Currently, this option is dynamically disabled if
combined with '--filter=<X>', even when using a simple filter such as
'blob:none' to signal a blobless packfile. This is a common scenario for
repos at scale, so is worth integrating.

Also, users can opt-in to the '--path-walk' option by default through
the pack.usePathWalk=true config option. When using that in a blobless
partial clone, the following warning can appear even though the user did
not specify either option directly:

  warning: cannot use --filter with --path-walk

Teach the path-walk API to handle the 'blob:none' object filter
natively. When revs->filter.choice is LOFC_BLOB_NONE, the path-walk
sets info->blobs to 0 (skipping all blob objects) and clears the
filter from revs so that prepare_revision_walk() does not reject the
configuration.

This check is implemented in the static prepare_filters() method, which
will simultaneously check if the input filters are compatible and will
make the appropriate mutations to the path_walk_info and filters if the
path_walk_info is non-NULL. This allows us to use this logic both in the
API method path_walk_filter_compatible() for use in
builtin/pack-objects.c and as a prep step in walk_objects_by_path().

Update the test helper (test-path-walk) to accept --filter=<spec>
as a test-tool option (before '--'), applying it to revs after
setup_revisions() to avoid the --objects requirement check. We can also
revert recent GIT_TEST_PACK_PATH_WALK overrides in t5620.

Also switch test-path-walk from REV_INFO_INIT with manual repo
assignment to repo_init_revisions(), which properly initializes
the filter_spec strbuf needed for filter parsing.

Add tests for blob:none with --all and with a single branch.

The performance test p5315 shows the impact of this change when using
blobless filters:

Test                                           HEAD~1     HEAD
---------------------------------------------------------------------
5315.6: repack (blob:none)                      13.53   13.87  +2.5%
5315.7: repack size (blob:none)                137.7M  137.8M  +0.1%
5315.8: repack (blob:none, --path-walk)         13.51   23.43 +73.4%
5315.9: repack size (blob:none, --path-walk)   137.7M  115.2M -16.3%

These performance tests were run on the Git repository. The --path-walk
feature shows meaningful space savings (16% smaller for blobless packs)
at the cost of increased computation time due to the two compression
passes. This data demonstrates that the feature is engaged and provides
real compression benefits when --no-reuse-delta forces fresh deltas.

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>