commands.update() now works properly with a revision of 0

Without this change commands.update() treats an integer 0 as if no revision
was passed and updates to the branch head.  This fix allows an integer 0 to
be detected as a revision number so the working directory is correctly
changed to revision 0 rather than the branch head.

mercurial/commands.py

@@ -4053,7 +4053,7 @@ def update(ui, repo, node=None, rev=None, clean=False, date=None, check=False):
     if rev and node:
         raise util.Abort(_("please specify just one revision"))
 
-    if not rev:
+    if rev is None or rev == '':
         rev = node
 
     # if we defined a bookmark, we have to remember the original bookmark name

tests/test-basic.t

@@ -20,6 +20,22 @@ This command is ancient:
   summary:     test
   
 
+Verify that updating to revision 0 via commands.update() works properly
+
+  $ cat <<EOF > update_to_rev0.py
+  > from mercurial import ui, hg, commands
+  > myui = ui.ui()
+  > repo = hg.repository(myui, path='.')
+  > commands.update(myui, repo, rev=0)
+  > EOF
+  $ hg up null
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ python ./update_to_rev0.py
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg identify -n
+  0
+ 
+
 Poke around at hashes:
 
   $ hg manifest --debug