Fix an odd revlog bug

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Fix an odd revlog bug

If revlog had a cached -empty- revision, as opposed to no cached
version, it could get confused. This cropped up in verify on a
particular repo.

manifest hash: 90ccf122087f6bbcb4322cb9d9bb8124610ba886
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.0 (GNU/Linux)

iD8DBQFCzjRaywK+sNU5EO8RAgVEAKCv3WBJt1rBOX0UlTDXFPygPIru+gCfTZxJ
CEz1lYny1gkQ+haGY26QdBs=
=C/K5
-----END PGP SIGNATURE-----

mercurial/revlog.py

@@ -248,7 +248,7 b' class revlog:'
         f.seek(start)
         data = f.read(end - start)
 
-        if not text:
+        if text is None:
             last = self.length(base)
             text = decompress(data[:last])