AmbiguousCommand is raised too soon.

Right now, hg raises AmbiguousCommand as soon as it finds two
commands/aliases that start with the substring it's searching for, even
though it may still find a full match later on.

This is a bit hard to hit on purpose, because hg checks the list of
commands in whatever order is returned by table.keys(), which will
change when you add an alias to a command.  You should be able to hit it
by adding an alias "u" to the "identify" command - not that that makes a
lot of sense...

mercurial/commands.py

@@ -2548,17 +2548,20 b' norepo = ("clone init version help debug'
 def find(cmd):
     """Return (aliases, command table entry) for command string."""
     choice = None
+    count = 0
     for e in table.keys():
         aliases = e.lstrip("^").split("|")
         if cmd in aliases:
             return aliases, table[e]
         for a in aliases:
             if a.startswith(cmd):
-                if choice:
-                    raise AmbiguousCommand(cmd)
-                else:
-                    choice = aliases, table[e]
-                    break
+                count += 1
+                choice = aliases, table[e]
+                break
+
+    if count > 1:
+        raise AmbiguousCommand(cmd)
+
     if choice:
         return choice