remotenames: move function to pull remotenames from the remoterepo to core This patch is the first patch of the series moving functionality from hgremotenames extension to core. There are lot of functionality in the extension which in the end enables us to store branch heads and bookmarks location on a server from which we are pulling or cloning from. This will help us in creating a better bookmark workflow where we can show user that a certain server has this bookmarks at this node. It will also introduce namespaces related to remote bookmarks and remote branches. This patch moves the functionality to pull branches and bookmarks from a server from which we are pulling to core behind config option `experimental.remotenames`. This patch adds a test which helps us to analyse whether things are working or not. We are currently writing things to ui, we will write information to files in upcoming patches. Previously reviewed as D937. Differential Revision: https://phab.mercurial-scm.org/D1547