configitems: register the 'format.usefncache' config

mercurial/configitems.py

@@ -125,6 +125,9 b" coreconfigitem('format', 'maxchainlen',"
 coreconfigitem('format', 'obsstore-version',
     default=None,
 )
+coreconfigitem('format', 'usefncache',
+    default=True,
+)
 coreconfigitem('hostsecurity', 'ciphers',
     default=None,
 )

mercurial/localrepo.py

@@ -2076,7 +2076,7 b' def newreporequirements(repo):'
     requirements = {'revlogv1'}
     if ui.configbool('format', 'usestore', True):
         requirements.add('store')
-        if ui.configbool('format', 'usefncache', True):
+        if ui.configbool('format', 'usefncache'):
             requirements.add('fncache')
             if ui.configbool('format', 'dotencode'):
                 requirements.add('dotencode')