[PATCH 1 of 3] update: fix "not rev" vs "rev is None"

[Patrick Mezard]

# HG changeset patch
# User Patrick Mezard <patrick at mezard.eu>
# Date 1335175794 -7200
# Branch stable
# Node ID 5e70ed4d13ffb1e931b05afc04affe66eb893480
# Parent  cbf2ea2f5ca169d22e0729cb71a21b808574b90e
update: fix "not rev" vs "rev is None"

diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -5664,7 +5664,7 @@
         c = repo[None]
         if c.modified() or c.added() or c.removed():
             raise util.Abort(_("uncommitted local changes"))
-        if not rev:
+        if rev is None:
             rev = repo[repo[None].branch()].rev()
         mergemod._checkunknown(repo, repo[None], repo[rev])