Fix typo that made change detection break

We were comparing the old and old mtimes rather than the old and
the new. This meant we missed some file changes where only the mtime
changed.

changes.go

@@ -119,7 +119,7 @@ func (info *FileInfo) addChanges(oldInfo *FileInfo, changes *[]Change) {
 			// breaks down is if some code intentionally hides a change by setting
 			// back mtime
 			oldMtime := syscall.NsecToTimeval(oldStat.Mtim.Nano())
-			newMtime := syscall.NsecToTimeval(oldStat.Mtim.Nano())
+			newMtime := syscall.NsecToTimeval(newStat.Mtim.Nano())
 			if oldStat.Mode != newStat.Mode ||
 				oldStat.Uid != newStat.Uid ||
 				oldStat.Gid != newStat.Gid ||