Don't seek at all if the log is small anyway

then it's fine to skip the first line because
it won't be very noticeable

lib/gitlab/ci/trace/stream.rb

@@ -25,12 +25,10 @@ module Gitlab
         end
         def limit(last_bytes = LIMIT_SIZE)
-          stream_size = size
-          if stream_size < last_bytes
-            last_bytes = stream_size
+          if last_bytes < size
+            stream.seek(-last_bytes, IO::SEEK_END)
+            stream.readline
           end
-          stream.seek(-last_bytes, IO::SEEK_END)
-          stream.readline
         end
         def append(data, offset)