configitems: register the 'worker.backgroundclosethreadcount' config

mercurial/configitems.py

@@ -161,3 +161,6 b" coreconfigitem('worker', 'backgroundclos"
 coreconfigitem('worker', 'backgroundcloseminfilecount',
     default=2048,
 )
+coreconfigitem('worker', 'backgroundclosethreadcount',
+    default=4,
+)

mercurial/vfs.py

@@ -550,7 +550,7 b' class backgroundfilecloser(object):'
         # Windows defaults to a limit of 512 open files. A buffer of 128
         # should give us enough headway.
         maxqueue = ui.configint('worker', 'backgroundclosemaxqueue')
-        threadcount = ui.configint('worker', 'backgroundclosethreadcount', 4)
+        threadcount = ui.configint('worker', 'backgroundclosethreadcount')
 
         ui.debug('starting %d threads for background file closing\n' %
                  threadcount)