ikhal: Fix deleting instances of recurring events

Before, after marking instances of recurring events as deleted, ikhal
would crash when trying to delete those events when exiting.

fixes pimutils#793

khal/khalendar/backend.py

@@ -575,6 +575,13 @@ def get(self, href: str, calendar: str) -> str:
         item, etag = self.sql_ex(sql_s, (href, calendar))[0]
         return item
 
+    def get_with_etag(self, href: str, calendar: str) -> Tuple[str, str]:
+        """returns the ical string and its etag matching href and calendar"""
+        assert calendar is not None
+        sql_s = 'SELECT item, etag FROM events WHERE href = ? AND calendar = ?;'
+        item, etag = self.sql_ex(sql_s, (href, calendar))[0]
+        return item, etag
+
     def search(self, search_string: str) \
             -> Iterable[Tuple[str, str, dt.date, dt.date, str, str, str]]:
         """search for events matching `search_string`"""

khal/khalendar/khalendar.py

@@ -224,9 +224,8 @@ def delete(self, href: str, etag: Optional[str], calendar: str):
 
     def get_event(self, href: str, calendar: str) -> Event:
         """get an event by its href from the datatbase"""
-        return self._construct_event(
-            self._backend.get(href, calendar), href=href, calendar=calendar,
-        )
+        event_str, etag = self._backend.get_with_etag(href, calendar)
+        return self._construct_event(event_str, etag=etag, href=href, calendar=calendar)
 
     def _construct_event(self,
                          item: str,