archive/tar: remove useless type conversions

Change-Id: I259a6ed6a1abc63d2dc39eca7e85f94cf38001cc
Reviewed-on: https://go-review.googlesource.com/47342Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/format.go

@@ -227,7 +227,7 @@ func (b *block) ComputeChecksum() (unsigned, signed int64) {
 		if 148 <= i && i < 156 {
 			c = ' ' // Treat the checksum field itself as all spaces.
 		}
-		unsigned += int64(uint8(c))
+		unsigned += int64(c)
 		signed += int64(int8(c))
 	}
 	return unsigned, signed

src/archive/tar/strconv.go

@@ -226,9 +226,9 @@ func parsePAXTime(s string) (time.Time, error) {
 	}
 	nsecs, _ := strconv.ParseInt(sn, 10, 64) // Must succeed
 	if len(ss) > 0 && ss[0] == '-' {
-		return time.Unix(secs, -1*int64(nsecs)), nil // Negative correction
+		return time.Unix(secs, -1*nsecs), nil // Negative correction
 	}
-	return time.Unix(secs, int64(nsecs)), nil
+	return time.Unix(secs, nsecs), nil
 }
 
 // formatPAXTime converts ts into a time of the form %d.%d as described in the