xonsh migrate history items

migrate.py

from xonsh.history.json import JsonHistory
from xonsh.history.sqlite import SqliteHistory

hist_json = JsonHistory(gc=False)

# if you want to have a check first
print('have {} json items'.format(len(list(hist_json.all_items()))))  # may very slow here

hist_db = SqliteHistory(gc=False)
for item in hist_json.all_items():
    if 'inp' not in item:
        continue
    if 'rtn' not in item:
        item['rtn'] = 0
    hist_db.append(item)