hg: note that islocal only accepts paths pointing to repos hg.islocal doesn't work for paths pointing to non-repos, such as patch files.

hg.openpath: use url.islocal to tell if the path is local (issue3624) Previously we used hg.islocal, which doesn't work for paths pointing to non-repos, such as patch files.

util.url: add an 'islocal' method This returns True if the URL represents a path that can be opened locally, without needing to go through the entire URL open mechanism.

push: move bookmarks exchange in the exchange module The bookmark exchange code was already extracted during a previous cycle. This changesets moves the extracted function in this module. This function will read and write data in the `pushoperation` object and It is preferable to have all core function collaborating through this object in the same place. This changeset is pure code movement only. Code change for direct consumption of the `pushoperation` object will come later.