pack-objects: support reachability bitmaps with `--path-walk`

pack-objects: support reachability bitmaps with `--path-walk`

When 'pack-objects' is invoked with '--path-walk', it prevents us from
using reachability bitmaps.

This behavior dates back to 70664d2865c (pack-objects: add --path-walk
option, 2025-05-16), which included a comment in the relevant portion of
the command-line arguments handling that read as follows:

    /*
     * We must disable the bitmaps because we are removing
     * the --objects / --objects-edge[-aggressive] options.
     */

In fb2c309b7d3 (pack-objects: pass --objects with --path-walk,
2026-05-02), path-walk learned to pass '--objects' again, but still
kept bitmap traversal disabled. That leaves two useful cases
unsupported:

 * A path-walk repack that writes bitmaps does not give the bitmap
   selector any commits, because path-walk reveals commits through
   `add_objects_by_path()` rather than through `show_commit()`, where
   `index_commit_for_bitmap()` is normally called.

 * An invocation like "git pack-objects --use-bitmap-index --path-walk"
   never tries an existing bitmap, even when one is available and could
   answer the request.

Fortunately for us, neither restriction is required.

 * On the writing side: teach the path-walk object callback to call
   `index_commit_for_bitmap()` for commits that it adds to the pack.
   That gives the bitmap selector the commit candidates it would have
   seen from the regular traversal.

 * For bitmap reading, keep passing '--objects' to the internal rev_list
   machinery, but stop clearing `use_bitmap_index`. If an existing
   bitmap can answer the request, use it; otherwise fall back to
   path-walk's own enumeration.

As a result, we can see significantly reduced pack sizes from p5311
before this commit:

    Test                                      HEAD^             HEAD
    ----------------------------------------------------------------------------------
    5311.38: server (1 days, --path-walk)     2.56(2.52+0.03)   0.01(0.01+0.00) -99.6%
    5311.39: size   (1 days, --path-walk)              123.9K            123.9K +0.0%
    5311.40: client (1 days, --path-walk)     0.00(0.01+0.00)   0.00(0.00+0.00) =
    5311.42: server (2 days, --path-walk)     2.57(2.52+0.05)   0.01(0.01+0.00) -99.6%
    5311.43: size   (2 days, --path-walk)              123.9K            123.9K +0.0%
    5311.44: client (2 days, --path-walk)     0.00(0.00+0.00)   0.00(0.00+0.00) =
    5311.46: server (4 days, --path-walk)     2.58(2.51+0.07)   0.01(0.01+0.00) -99.6%
    5311.47: size   (4 days, --path-walk)              123.9K            123.9K +0.0%
    5311.48: client (4 days, --path-walk)     0.00(0.00+0.00)   0.00(0.00+0.00) =
    5311.50: server (8 days, --path-walk)     2.58(2.53+0.04)   0.02(0.02+0.00) -99.2%
    5311.51: size   (8 days, --path-walk)              152.4K            152.4K +0.0%
    5311.52: client (8 days, --path-walk)     0.00(0.01+0.00)   0.00(0.01+0.00) =
    5311.54: server (16 days, --path-walk)    2.58(2.52+0.05)   0.03(0.02+0.00) -98.8%
    5311.55: size   (16 days, --path-walk)             205.3K            205.3K +0.0%
    5311.56: client (16 days, --path-walk)    0.01(0.01+0.00)   0.01(0.01+0.00) +0.0%
    5311.58: server (32 days, --path-walk)    2.59(2.53+0.06)   0.03(0.03+0.00) -98.8%
    5311.59: size   (32 days, --path-walk)             209.3K            209.3K +0.0%
    5311.60: client (32 days, --path-walk)    0.01(0.02+0.00)   0.01(0.02+0.00) +0.0%
    5311.62: server (64 days, --path-walk)    2.70(2.76+0.06)   0.16(0.24+0.04) -94.1%
    5311.63: size   (64 days, --path-walk)               4.1M              4.1M +0.0%
    5311.64: client (64 days, --path-walk)    0.44(0.50+0.02)   0.44(0.51+0.02) +0.0%
    5311.66: server (128 days, --path-walk)   2.88(3.20+0.05)   0.34(0.65+0.05) -88.2%
    5311.67: size   (128 days, --path-walk)              9.0M              9.0M -0.0%
    5311.68: client (128 days, --path-walk)   0.93(1.22+0.07)   0.93(1.20+0.08) +0.0%

We get the same size of output pack, but this commit allows us to do so
in a significantly shorter amount of time. Intuitively, we're generating
the same pack (hence the unchanged 'test_size' output from run to run),
but varying how we get there. Before this commit, pack-objects prefers
'--path-walk' to '--use-bitmap-index', so we generate the output pack by
performing a normal '--path-walk' traversal. With this commit, we are
operating over a *repacked* state (that itself was done with a
'--path-walk' traversal), but are able to perform pack-reuse on that
repacked state via bitmaps.

There is one wrinkle when it comes to '--boundary', which we must not
pass into the bitmap walk in the presence of both '--path-walk' and
'--use-bitmap-index'. Path-walk needs boundary commits when it performs
its own traversal, in order to discover bases for thin packs, but the
bitmap traversal does not expect this. Work around this by setting
`revs->boundary` as late as possible within the '--path-walk' traversal,
after any bitmap attempt has either succeeded or declined to answer the
request.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>