Modify _last_history function

The old method, using a many2one and _read_flat(), returned the wrong date if there were more than 1000 history lines.
Better to put the compact form:
```
result[reconcile_id] = len(last_history) and last_history[0] or False
```
or with a ternary operator:
```
result[reconcile_id] = last_history[0] if len(last_history) else False
```