narrow: introduce a config option to check if narrow is enabled or not This patch introduces a new config option experimental.narrow which is set to False by default and set to True by the narrow extension. While moving narrow related logic into core, we need to know at places whether narrow extension is enabled or not. Checking the list of extension enabled is one solution but once narrow is inbuilt, we will definitely want a config option to check whether narrow is turned on or not. So this patch introduces a config option, which will evolve to the main point to turn narrow capability on and off once all the narrow is in core. Differential Revision: https://phab.mercurial-scm.org/D4889