namespaces: add tags

namespaces: remove weakref; always pass in repo It turns out that maintaining a reference of any sort (even weak!) to the repo when constructed doesn't work because we may at some point pass in a repoview filtered by something other than what the initial repo was.

changectx: use names api to simplify and extend node lookup Previously, changectx had to know about each type of name (bookmark, tag, etc.) to look up. Now, we use repo.namenodes to simplify (and extend) this.

namespaces: add a method to the first matching node for a given name

namespaces: add bookmarks to the names data structure This marks the first use of abstracting our different types of named objects (bookmarks, tags, branches, etc.) and upcoming patches will use this to simplify logic.

namespaces: add a convenience property for the weakref _repo

namespaces: pass repo to __init__ We store repo as a weakref so that we can prevent a cycle between namespaces and localrepo.

namespaces: add a function to turn single results into lists