merge: notice uncommited copies (issue1000)

As mentioned in msg5349 in issue988, "hg update" doesn't take into account
uncommitted copies.  To reproduce:

----------------
hg init repo
cd repo

touch foo
hg ci -Am 'add foo'

echo >> foo
hg ci -m 'change foo'

hg up -C 0
hg mv foo bar
HGMERGE=false thg --debug -y update
--------------------------

A similar problem happens with hg merge --force.

I'm attaching a possible patch.

mercurial/merge.py

@@ -106,7 +106,7 b' def findcopies(repo, m1, m2, ma, limit):'
             seen[s] = 1
             if fc.path() != orig and fc.path() not in old:
                 old[fc.path()] = 1
-            if fc.rev() < limit:
+            if fc.rev() < limit and fc.rev() is not None:
                 continue
             visit += fc.parents()