git add -A resolves the index U-stage regardless of file content, so a
conflicted file whose markers were never removed still got staged and
committed as-is. Check previously-conflicted files for leftover
<<<<<<< markers before staging; refuse with a conflict result if any
remain, leaving the repo mid-merge so Abort still cleans up.
Adds a regression test that leaves one conflict unresolved and asserts
ContinueMergeAsync refuses (no commit) and AbortMergeAsync still
restores a clean tree.
mika kuns
377409e633