run-tests.py: fix diff output when test-foo.out doesn't exist.
Previously, if test-foo.out doesn't exist and test-foo outputs
something, the first line of the diff will look like this:
-+changeset: 0:0acdaf898367
(note the leading "-")
#!/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")