Fix trip summary line in dive list

Two errors fixed.

- With no location set, the summary line would start with a ','.
- When auto creating a trip for a manually added dive or when editing the
  dates of dives in a trip, the timestamp for the trip was not updated
  after editing the dive.

Fixes #293

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

divelist.h

@@ -30,6 +30,7 @@ extern struct dive *merge_two_dives(struct dive *a, struct dive *b);
 extern bool consecutive_selected();
 extern void select_dive(int idx);
 extern void deselect_dive(int idx);
+void find_new_trip_start_time(dive_trip_t *trip);
 
 #ifdef DEBUG_TRIP
 extern void dump_selection(void);