remotenames: add functionality to hoist remotebookmarks
This patch adds the functionality to hoist remotebookmarks to the top level
namespace. The peer of which bookmarks should be hoisted can be set using
`remotenames.hoistedpeer` config option. Only bookmarks can be hoisted. If a
hoisted name and local bookmark exists of the same name, the local bookmark
takes precedence.
While I was here, I documented the default values of two other remotenames
config options.
Differential Revision: https://phab.mercurial-scm.org/D2807