graft: explicit current node tracking

This changes graft to explicitly track the progression of commits it
makes, and updates it's idea of the current node based on it's last
commit, rather than from the working copy parent.  This should have no
effect on the value of current since we were reading the working copy
parent immediately after commiting to it.

The motivation for this change is that a subsequent patch will break
the current node and working copy relationship.  Splitting this out
into a separate patch will make that one more readible.

mercurial/commands.py

@@ -2830,8 +2830,8 b' def graft(ui, repo, *revs, **opts):'
 
     wlock = repo.wlock()
     try:
+        current = repo['.']
         for pos, ctx in enumerate(repo.set("%ld", revs)):
-            current = repo['.']
 
             ui.status(_('grafting revision %s\n') % ctx.rev())
             if opts.get('dry_run'):
@@ -2883,6 +2883,8 b' def graft(ui, repo, *revs, **opts):'
                         date=date, extra=extra, editor=editor)
             if node is None:
                 ui.status(_('graft for revision %s is empty\n') % ctx.rev())
+            else:
+                current = repo[node]
     finally:
         wlock.release()