alias: test duplicated definition earlier

This patch moves the old definition checking logic introduced by
f4b7be3f8430 earlier. So that the test itself does not depend on `aliasdef`.

The check is to avoid wrapping a same alias multiple times. It can be done
by checking the config name and value (`definition` in code), without
constructing a `cmdalias` instance.

This makes the next patch easier to review.

Differential Revision: https://phab.mercurial-scm.org/D804

mercurial/dispatch.py

@@ -528,17 +528,15 b' def addaliases(ui, cmdtable):'
     # may use extension commands. Aliases can also use other alias definitions,
     # but only if they have been defined prior to the current definition.
     for alias, definition in ui.configitems('alias'):
-        source = ui.configsource('alias', alias)
-        aliasdef = cmdalias(alias, definition, cmdtable, source)
-
         try:
-            olddef = cmdtable[aliasdef.cmd][0]
-            if olddef.definition == aliasdef.definition:
+            if cmdtable[alias][0].definition == definition:
                 continue
         except (KeyError, AttributeError):
             # definition might not exist or it might not be a cmdalias
             pass
 
+        source = ui.configsource('alias', alias)
+        aliasdef = cmdalias(alias, definition, cmdtable, source)
         cmdtable[aliasdef.name] = (aliasdef, aliasdef.opts, aliasdef.help)
 
 def _parse(ui, args):