forget: use vfs instead of os.path + match.rel() for filesystem checks

hgext/largefiles/overrides.py

@@ -971,7 +971,7 b' def overrideforget(orig, ui, repo, *pats'
 
     for f in forget:
         if lfutil.standin(f) not in repo.dirstate and not \
-                os.path.isdir(m.rel(lfutil.standin(f))):
+                repo.wvfs.isdir(lfutil.standin(f)):
             ui.warn(_('not removing %s: file is already untracked\n')
                     % m.rel(f))
             result = 1

mercurial/cmdutil.py

@@ -2037,9 +2037,9 b' def forget(ui, repo, match, prefix, expl'
 
     if not explicitonly:
         for f in match.files():
-            if f not in repo.dirstate and not os.path.isdir(match.rel(join(f))):
+            if f not in repo.dirstate and not repo.wvfs.isdir(f):
                 if f not in forgot:
-                    if os.path.exists(match.rel(join(f))):
+                    if repo.wvfs.exists(f):
                         ui.warn(_('not removing %s: '
                                   'file is already untracked\n')
                                 % match.rel(join(f)))