dirstate-item: use `set_possibly_dirty` in `pure.pack_dirstate`

Lets make use of the new function.

Differential Revision: https://phab.mercurial-scm.org/D11120

mercurial/pure/parsers.py

@@ -588,8 +588,7 b' def pack_dirstate(dmap, copymap, pl, now'
             # dirstate, forcing future 'status' calls to compare the
             # contents of the file if the size is the same. This prevents
             # mistakenly treating such files as clean.
-            e = DirstateItem(e.state, e.mode, e.size, AMBIGUOUS_TIME)
-            dmap[f] = e
+            e.set_possibly_dirty()
 
         if f in copymap:
             f = b"%s\0%s" % (f, copymap[f])