amend: Preserve amended commit message on stack replay conflict

[Abhishek21g]

Summary

Fixes #266.

When revup amend rewrites a commit message and then fails while replaying the rest of the stack (e.g. merge conflict), the user's new message is now saved under .revup/AMENDED_COMMIT_MSG in the scratch directory and the path is logged. The message is only saved when it actually changed from the original commit.

Repro

1. Create a stack where amending an earlier commit conflicts with a later commit on the same file
2. Run revup amend HEAD~1 with --edit and a new message
3. On conflict, previously the amended message was lost

Fix

• Add preserve_amended_commit_msg() called from amend conflict paths
• Only persist when amended_msg != original_msg

Verification

python3 -m pip install -e '.[dev]'
python3 -m pytest tests/test_amend.py::TestAmendConflict -q
make lint