maintenance(geometric): avoid deadlocks on Windows 10 (#6215)

This PR is a companion of gitgitgadget#2103.

On Windows, `maintenance_task_geometric_repack()` opens pack index files
via `pack_geometry_init()` (which `mmap()`s the `.idx` files), then
spawns `git repack` as a child process without setting
`child.odb_to_close`. The parent's `mmap()`s prevent the child from
deleting old `.idx` files.

On Windows 10 builds before the POSIX delete semantics change (between
Build 17134.1304 and 18363.657, see
https://stackoverflow.com/a/60512798), this results in `Unlink of file
'.git/objects/pack/pack-<hash>.idx' failed. Should I try again?` during
fetch-triggered auto-maintenance with the geometric strategy.

The fix adds the missing `child.odb_to_close = the_repository->objects`
line, matching all other maintenance tasks.

The first commit introduces a `GIT_TEST_LEGACY_DELETE` environment
variable to simulate legacy (pre-POSIX) delete semantics on modern
Windows, so the regression test can verify the fix even on Windows 11.

This fixes #6210.

Tested-by: Patryk Miś <foss@patrykmis.com>

Author: dscho