Fix message with actual diff file name

The message said "/db/diffs/changes.osm exists", but the file name is in
$DIFF_FILE.

Author: greenscientist

bin/update_overpass.sh

@@ -52,7 +52,7 @@ fi
 				set -e
 			fi
 		else
-			echo "/db/diffs/changes.osm exists. Trying to apply again."
+			echo "${DIFF_FILE} exists. Trying to apply again."
 		fi
 
 		# if DIFF_FILE is non-empty, try to process it