avoid calling (cd ...) with `nil' as a directory name
When (hg-root) cannot determine the current Mercurial root, print an
error message instead of trying to evaluate (cd nil).
#!/usr/bin/env python
import sys, os
from mercurial import hg
f = sys.argv[1]
r1 = hg.revlog(open, f + ".i", f + ".d")
r2 = hg.revlog(open, f + ".i2", f + ".d2")
tr = hg.transaction(open, "journal")
for i in xrange(r1.count()):
n = r1.node(i)
p1, p2 = r1.parents(n)
l = r1.linkrev(n)
t = r1.revision(n)
n2 = r2.addrevision(t, tr, l, p1, p2)
tr.close()
os.rename(f + ".i", f + ".i.old")
os.rename(f + ".d", f + ".d.old")
os.rename(f + ".i2", f + ".i")
os.rename(f + ".d2", f + ".d")