archive/tar: optimize formatter's formatString

When dealing with the last special case,
we only need to know the string length after TrimRight '/'.
This optimization clarifies what we really want and speeds up the execution.

src/archive/tar/strconv.go

@@ -73,8 +73,13 @@ func (f *formatter) formatString(b []byte, s string) {
 	// in the V7 path field as a directory even though the full path
 	// recorded elsewhere (e.g., via PAX record) contains no trailing slash.
 	if len(s) > len(b) && b[len(b)-1] == '/' {
-		n := len(strings.TrimRight(s[:len(b)], "/"))
-		b[n] = 0 // Replace trailing slash with NUL terminator
+		i := len(b) - 2 // s[len(b)-1] == '/'
+		for ; i >= 0; i-- {
+			if s[i] != '/' {
+				break
+			}
+		}
+		b[i+1] = 0 // Replace trailing slash with NUL terminator
 	}
 }