Fixed existing diary was not found correctly.

In ec1d245bfa8d66027dad2d8ba48ecc6e70674fce , format of a filename was
changed. However, the function calendar#sign was not changed, thus the
function was not able to find diary files. This commit fixes the search
path.
Merge pull request #18 from KainokiKaede/master

Fix existing diary was not found correctly.