git-lfs: remember urls, and autoenable remotes using known urls

* git-lfs: The url provided to initremote/enableremote will now be
  stored in the git-annex branch, allowing enableremote to be used without
  an url. initremote --sameas can be used to add additional urls.
* git-lfs: When there's a git remote with an url that's known to be
  used for git-lfs, automatically enable the special remote.

Git/Config.hs

@@ -94,6 +94,14 @@ store s repo = do
 		, fullconfig = M.unionWith (++) c (fullconfig repo)
 		}
 
+{- Stores a single config setting in a Repo, returning the new version of
+ - the Repo. Config settings can be updated incrementally. -}
+store' :: String -> String -> Repo -> Repo
+store' k v repo = repo
+	{ config = M.singleton k v `M.union` config repo
+	, fullconfig = M.unionWith (++) (M.singleton k [v]) (fullconfig repo)
+	}
+
 {- Updates the location of a repo, based on its configuration.
  -
  - Git.Construct makes LocalUknown repos, of which only a directory is