largefiles: cache new largefiles for new heads when pulling When the user pulls from a remote repository that is not his default repo, it is quite likely that he will pull a new head. This means that if he tries to merge or rebase with the other head, he will run into a problem becuase largefiles has no way of tracking where the remote repository for this other head is, so it cannot download the largefiles from this other remote repository. It will attempt to download them from its default remote repository, which will not yet contain the largefiles. This patch solves this problem by caching any new largefiles for all heads directly into the system cache at the time of the pull, so they are available later. This behavior is actually more in line with Mercurial's distributed nature, because pulling already implies we have a connection to the remote server, but merging or rebasing does not.