largefiles: tiny code clean up

lfutil.islfilesrepo is a preffered way for testing if repo is largefiles enabled

hgext/largefiles/overrides.py

@@ -63,7 +63,7 b' def override_add(orig, ui, repo, *pats, '
         ui, lfutil.islfilesrepo(repo), opts.pop('lfsize', None))
 
     lfmatcher = None
-    if os.path.exists(repo.wjoin(lfutil.shortname)):
+    if lfutil.islfilesrepo(repo):
         lfpats = ui.configlist(lfutil.longname, 'patterns', default=[])
         if lfpats:
             lfmatcher = match_.match(repo.root, '', list(lfpats))