revlog: add exception when linkrev == nullrev

When we deployed the latest crew mercurial to our users, a few of them
had issues where a filelog would have an entry with a -1 linkrev. This
caused operations like rebase and amend to create a bundle containing the
entire repository, which took a long time.

I don't know what the issue is, but adding this check should prevent repos
from getting in this state, and should help us pinpoint the issue next time
it happens.

mercurial/revlog.py

@@ -991,6 +991,9 @@ class revlog(object):
         p1, p2 - the parent nodeids of the revision
         cachedelta - an optional precomputed delta
         """
+        if link == nullrev:
+            raise RevlogError(_("attempted to add linkrev -1 to %s")
+                              % self.indexfile)
         node = hash(text, p1, p2)
         if node in self.nodemap:
             return node