mergeTaskBranches and applyNestedPatches both stashed dirty WIP, cherry-picked
task branches, then called `git stash pop` in a finally block. On conflict git
left stage 1/2/3 unmerged entries in .git/index with no MERGE_HEAD to abort;
neither call cleaned up. The corrupted index persisted indefinitely, and every
subsequent overlay-isolated task inherited it through the lower layer —
captureRepoDeltaPatch then emitted `diff --cc` (combined merge format) that
git apply rejects with "No valid patches in input", failing every downstream
task merge with 'Branch merge failed before a task branch could be created'.
Fix at the git API level: git.stash.tryPop now runs `git apply --3way --check`
on `git stash show -p --binary stash@{0}` before popping (`--3way` matches
what git stash pop does internally, so context that drifted after cherry-pick
is still accepted). Preflight failure short-circuits — stash entry preserved,
index untouched. Preflight pass falls through to pop; if pop still leaves
unmerged entries (mode-only or delete/modify conflicts the preflight can miss),
a `reset --hard HEAD" fallback restores the merged HEAD without losing the
cherry-picked commits (stash is preserved by git on failed pop, so the user's
WIP stays recoverable).
Both call sites now share this contract via git.stash.tryPop.
Fixes#4175
roboomp
ffd6a57d2f