contrib/subtree: fix split with squashed subtrees

contrib/subtree: fix split with squashed subtrees

98ba49ccc2 (subtree: fix split processing with multiple subtrees
present, 2023-12-01) increases the performance of

    git subtree split --prefix=subA

by ignoring subtree merges which are outside of `subA/`. It also
introduces a regression. Subtree merges that should be retained
are incorrectly ignored if they:

1. are nested under `subA/`; and
2. are merged with `--squash`.

For example, a subtree merged like:

    git subtree merge --squash --prefix=subA/subB "$rev"
    #                 ^^^^^^^^          ^^^^

is erroneously ignored during a split of `subA`. This causes
missing tree files and different commit hashes starting in
git v2.44.0-rc0.

The method:

    should_ignore_subtree_split_commit REV

should test only a single commit REV, but the combination of

    git log -1 --grep=...

actually searches all *parent* commits until a `--grep` match is
discovered.

Rewrite this method to test only one REV at a time. Extract commit
information with a single `git` call as opposed to three. The
`test` conditions for rejecting a commit remain unchanged.

Unit tests now cover nested subtrees.

Signed-off-by: Colin Stagner <ask+git@howdoi.land>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>