Re: [Bug] Git subtree regression

[george@mail.dietrich.pub]

---
I explored this more and think I found the root cause.
Commit `83f9dad7d6fb5988b68f80b25bd87c68693195dd` changed `should_ignore_subtree_split_commit()` to examine only a commit's own trailers via `git show --format='%(trailers:...)'`.
The old code used `git log -1 --grep=...` which had the important side effect of searching through ancestor commits.

In a multi-subtree monorepo with this topology:

```
  main:    A---B---M---E    (B = subtree add --squash for subA)
                  /
  feature:   C---D          (D = subtree add --squash for subB)
```

When splitting `subA`, commits `C` and `D` from the feature branch should be **ignored** because they belong to a branch that only contains `subB`, not `subA`.

## Old behavior (2.51.1)

`git log -1 --grep="git-subtree-dir:"` on commit `C` would traverse ancestors and find `D`'s subtree markers for `subB`, correctly identifying `C` as belonging to another subtree's branch.

## New behavior (2.52.0)

`git show` on commit `C` finds no trailers (regular commits don't have them), so `C` is **not** ignored.
This breaks the split because both parents of the merge `M` are processed, but `C` has no cache entry, leading to disconnected history.
Thus, split operations produce fewer commits than expected with broken parent chains, breaking push/pull workflows to upstream subtree repositories.

## Reproduction

This can be reproduced via my monorepo: https://github.com/athena-framework/athena

```bash
# 2.51.1 produces correct result (matches the number of commits in the `athena-framework/clock` repo)
$ git subtree split --prefix="src/components/clock"
4ee66f8198b2532110b75a36575e363ccccff47e  # 20 commits, connected to remote

# 2.52.0 produces broken result
$ git subtree split --prefix="src/components/clock"
0efb3d9858e3bfee65165508aeeacc50417c9a99  # 7 commits, disconnected

# The commits have identical trees but different parents:
$ git cat-file -p 4ee66f8198b2532110b75a36575e363ccccff47e
tree 8333b0cbb2a10528f8c803812af7a8e603e70367
parent d72f22f28ca5ed57ef3c2df74f0abd5569ac5934  # Connected to 19-commit history

$ git cat-file -p 0efb3d9858e3bfee65165508aeeacc50417c9a99
tree 8333b0cbb2a10528f8c803812af7a8e603e70367
parent 81c5dbe70ce26a7758fbe7f87b3ce0704043cfb1  # Only 6 commits, disconnected
```