largefile: use `set_untracked` in the `forget` override

This is the new shinny API.

Differential Revision: https://phab.mercurial-scm.org/D11110

hgext/largefiles/overrides.py

@@ -1380,10 +1380,7 b' def cmdutilforget('
     with repo.wlock():
         lfdirstate = lfutil.openlfdirstate(ui, repo)
         for f in forget:
-            if lfdirstate[f] == b'a':
-                lfdirstate.drop(f)
-            else:
-                lfdirstate.remove(f)
+            lfdirstate.set_untracked(f)
         lfdirstate.write()
         standins = [lfutil.standin(f) for f in forget]
         for f in standins: