show: also catch AmbiguousCommand

cmdutil.findcmd() can raise this as well. While we'll almost certainly
never encounter this in the wild, guard against it regardless.

hgext/show.py

@@ -233,7 +233,7 b' def showstack(ui, repo, displayer):'
     try:
         cmdutil.findcmd('rebase', commands.table)
         haverebase = True
-    except error.UnknownCommand:
+    except (error.AmbiguousCommand, error.UnknownCommand):
         haverebase = False
 
     # TODO use templating.