configitems: restore alias for format.aggressivemergedeltas

913ca175c4ae broke BC by dropping support for reading
format.aggressivemergedeltas. Let's restore it.

Differential Revision: https://phab.mercurial-scm.org/D3966

mercurial/configitems.py

@@ -929,7 +929,7 b" coreconfigitem('push', 'pushvars.server'"
 )
 coreconfigitem('revlog', 'optimize-delta-parent-choice',
     default=True,
-    # formely an experimental option: format.aggressivemergedeltas
+    alias=[('format', 'aggressivemergedeltas')],
 )
 coreconfigitem('server', 'bookmarks-pushkey-compat',
     default=True,