The cost of taking out a 30-year fixed-rate mortgage edged up slightly this week, while rates on 15-year loans popular with borrowers who are refinancing registered an almost imperceptible drop.Both fixed-rate and adjustable-rate mortgage (ARM) loans remain at or near historic lows, with rates on 1-year Treasury-indexed ARM loans hitting a new low, Freddie Mac said in releasing the results of its latest Primary Mortgage Market Survey.Rates on 30-year fixed-rate mortgages averaged 3.37 percent with an average 0.7 point for the week ending Dec. 20, up from 3.32 percent last week but down from 3.91 percent a year ago. Rates on 30-year fixed-rate loans hit a low in Freddie Mac records dating to 1971 of 3.31 percent during the week ending Nov. 21.For 15-year fixed-rate loans, rates averaged 2.6...