dirstate: clarify comment about leaving normal files undef if changed 'now'

Clarify that they only are saved as undef if they were marked as normal and
changed in the same second.

mercurial/dirstate.py

@@ -882,9 +882,9 b' class dirstate(object):'
                 elif time != mtime and time != mtime & _rangemask:
                     ladd(fn)
                 elif mtime == lastnormaltime:
-                    # fn may have been changed in the same timeslot without
-                    # changing its size. This can happen if we quickly do
-                    # multiple commits in a single transaction.
+                    # fn may have just been marked as normal and it may have
+                    # changed in the same second without changing its size.
+                    # This can happen if we quickly do multiple commits.
                     # Force lookup, so we don't miss such a racy file change.
                     ladd(fn)
                 elif listclean: