sharesafe: introduce functionality to automatically upgrade shares In past few months, we have developed a `share-safe` mode for sharing repository in which share source requirements and config values are shared with the shares. To get it rolling, an important task is to get these shares automatically upgraded. We are focusing on an installation where shares are created by scripts and test jobs. It will be difficult to manually upgrade these and we need some functionality to do so automatically. This patch introduces a config option to deal with it. If all of the following conditions are met, we upgrade the share repository automatically: * If the config option is enabled * Share source repository is share-safe enabled * Share is not share-safe enabled * Any command is run in the share Upgrading the share is pretty easy as it involves only editing the requirements file. Differential Revision: https://phab.mercurial-scm.org/D9679