[client] Fix a bug in merge.

Reproduce sequence (only for version 0 repo):
1. Both client and server change the same file;
2. Client syncs and do a real merge and fails with conflict;
3. The 30-second auto syncing timer kicks in, sync state [initializing] -> [merging];
4. Merge returns "Already up to date".
5. Afterwards, every sync of this repo will stuck in "Already up to date".