[PATCH 0 of 1] [resend] transplant: update transplants file when nonexistent and when a new changegroup arrives

[Georg Brandl]

Resend again, with two changes:

* Also rebuild transplants file when it does not exist; this is necessary
  to get an initial cache for local clones and repositories created before
  the transplant extension is activated.

(An extension to this would be to change the format or call the file
differently, to catch the cases of partial transplants files before this
changeset is introduced.)

* Better method for registering the hook: call ui.setconfig() in uisetup.