enable reusing records in csv/tsv detection (#443)

This change should help with allocations, but ideally there would be no
allocations when detecting csv. For that to happen the detection needs
to move from using stdlib csv reader to something else.

internal/magic/text_csv.go

@@ -20,7 +20,7 @@ func Tsv(raw []byte, limit uint32) bool {
 func sv(in []byte, comma rune, limit uint32) bool {
 	r := csv.NewReader(dropLastLine(in, limit))
 	r.Comma = comma
-	r.TrimLeadingSpace = true
+	r.ReuseRecord = true
 	r.LazyQuotes = true
 	r.Comment = '#'