merge_trees: identical add/add on both sides reported as Err(Unknown)

[jonathanmorley]

Identical add/add on both sides reported as Err(Unknown) in multi-file tree merge

Crate: gix-merge 0.20.1 (via gix 0.87.1)

When both sides of a three-way tree merge add the same path with the
byte-identical blob, merge_trees reports the entry as Err(Unknown)
instead of auto-resolving. Git merges the identical inputs cleanly.

Observed entry

ours=Addition {
  location: "examples/resources/repository_private_vulnerability_reporting/example_1.tf",
  entry_mode: 100644,
  id: 049444c2236ddc48c12ac711498a17fb04629d83,   // same blob
}
theirs=Addition {
  location: "examples/resources/repository_private_vulnerability_reporting/example_1.tf",
  entry_mode: 100644,
  id: 049444c2236ddc48c12ac711498a17fb04629d83,   // same blob
}
resolution=Err(Unknown)

Full-fidelity reproduction (git side is clean)

All SHAs are public and immutable (jonathanmorley/terraform-provider-github, branches sharing history with integrations/terraform-provider-github@main):

git clone --bare https://github.com/jonathanmorley/terraform-provider-github.git repro.git
cd repro.git
git fetch https://github.com/integrations/terraform-provider-github.git \
  "refs/heads/main:refs/heads/upstream-main"

# ancestor = merge-base, ours = patch-0 tip, theirs = patch-1 tip:
git merge-tree 3ac346fdfbee2e19589a6203b20d936189ba0760 \
  f1beca348f4beb80688c0046aec6cbfb4c435596 \
  2b8bcab30cc83260154dce6a885a29cb17e6ffcf
# => exit 0, no conflict markers

The same ancestor/ours/theirs through Repository::merge_trees (default
tree_merge_options) yields the Err(Unknown) above (plus an auto-resolved
content merge, which is expected per the Outcome::conflicts docs and is
not part of this report).

What was ruled out

• Single-file identical-add is clean. Ancestor empty (or holding an
  unrelated file), both sides adding the same file and blob: zero conflicts
  listed, with and without rename tracking. The trigger needs multi-file
  context — the live entries carried differing ChildOfParent directory
  relations ((2) vs (3)), suggesting directory-level rename pairing
  sweeps the agreed file into Unknown.
• Rename toggle is not the (whole) story. Options::with_rewrites(None)
  does not change the outcome on the live inputs.
• Not this report: auto-resolved entries appearing in conflicts is
  documented behavior; only the identical-add Err(Unknown) is reported.

Repro materials

Full writeup, the three adjacent blob states, and exact commands:
https://github.com/jonathanmorley/reproductions/tree/gix/merge-identical-add-unknown

Minimal trigger: open

A single-file synthetic repro does not trigger it; the exact minimal
conditions (tree size? directory similarity? relation state?) are
unestablished. Happy to help minimize further given pointers on where to
look in the rename/conflict-classification path.