repoview: protect `base` computation from weird phase root

If for some reason the phase roots contains nullid, the set of filtered revs
will contains -1. That confuse Mercurial a lot. In particular this corrupt the
branchcache.

Standard code path does not result in nullid phase root. It can only result from
altered `.hg/store/phaseroots` or buggy extension. However better safe than
sorry.

mercurial/repoview.py

@@ -90,6 +90,8 b' def computeimpactable(repo):'
     for roots in repo._phasecache.phaseroots[1:]:
         if roots:
             firstmutable = min(firstmutable, min(cl.rev(r) for r in roots))
+    # protect from nullrev root
+    firstmutable = max(0, firstmutable)
     return frozenset(xrange(firstmutable, len(cl)))
 
 # function to compute filtered set