This is an object that is owned by Gui, is accessible through GuiCommon.State(),
and also passed down to GitCommand, where it is mostly needed. Right now it
simply wraps access to the Git.Paging config, which isn't very exciting, but
we'll extend it in the next commit to handle a slice of pagers (and maintain the
currently selected pager index), and doing this refactoring up front allows us
to make that change without having to touch clients.