workers: add config to enable/diable workers This adds config to disable/enable workers with default being enabled. Test Plan: enabled profile without updaing .hg/hgrc (the default should be to use workers) and ran hg sprase --enable-profile <profile>.sparse Watched in the proces explorer that hg started 12 new threads for materializing files (this is my worker.numcpus) value Added [worker] enabled = False to the .hg/hgrc and re ran the command. This time hg didn't spawn any new threads for matreializing of files Differential Revision: https://phab.mercurial-scm.org/D1460

--- a/mercurial/worker.py	Mon Nov 20 10:27:41 2017 -0800
+++ b/mercurial/worker.py	Sun Dec 17 11:26:25 2017 -0800
@@ -82,7 +82,8 @@
     args - arguments to split into chunks, to pass to individual
     workers
     '''
-    if worthwhile(ui, costperarg, len(args)):
+    enabled = ui.configbool('worker', 'enabled')
+    if enabled and worthwhile(ui, costperarg, len(args)):
         return _platformworker(ui, func, staticargs, args)
     return func(*staticargs + (args,))