contrib/subtree: process out-of-prefix subtrees

`should_ignore_subtree_split_commit` detects subtrees which are
outside of the current path --prefix and ignores them. This can
speed up splits of repositories that have many subtrees.

Since its inception [1], every iteration of this logic [2], [3]
incorrectly excludes commits. This alters the split history. The
split history and its commit hashes are API contract, so this is
not permissible.

While a commit from a different subtree may look like it doesn't
contribute anything to a split, sometimes it does. Merge commits
are a particular hot spot. For these, the pruning logic in
`copy_or_skip` performs:

1. a check for "treesame" parents
2. two different common ancestry checks

These checks operate on the **split history**, not the input
history. The split history omits commits that do not affect the
--prefix. This can significantly alter the ancestry of a merge.
In order to determine if `copy_or_skip` will skip a merge, it
is likely necessary to compute all the split history... which
is what `should_ignore_subtree_split_commit` tries to avoid.

To make this logic API-preserving, we could gate it behind a
new CLI argument. The present implementation is actually a
speed penalty in many cases, however, so this is not done here.

Remove the `should_ignore_subtree_split_commit` logic. This
fixes the regression reported in [4].

[1]: 98ba49ccc2 (subtree: fix split processing with multiple subtrees present, 2023-12-01)

[2]: 83f9dad7d6 (contrib/subtree: fix split with squashed subtrees, 2025-09-09)

[3]: 28a7e27cff (contrib/subtree: detect rewritten subtree commits, 2026-01-09)

[4]: <20251230170719.845029-1-george@mail.dietrich.pub>

Reported-by: George <george@mail.dietrich.pub>
Reported-by: Christian Heusel <christian@heusel.eu>
Signed-off-by: Colin Stagner <ask+git@howdoi.land>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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