Two problems of completion

[Herbert Euler]

Currently SPC is bound to minibuffer-complete-word in minibuffer.
With partial completion facilities implemented for minibuffer, I don't
know whether it is still right to bind SPC to this command, but if so
I find two problems.

1. Special case: the string before completion is the empty string "".

In this case, completion--try-word-completion will see a completion
available, i.e. ("" . 0), and then try firstly completing with " " (""
+ " ") and then "-" ("" + "-").  Partial completion often (e.g. in a
command execution context, i.e. typing M-x then typing SPC) returns a
result of ("-" . 0), make a completion available.

I think when the user press SPC directly in such a case, it means that
the user wants to see a list of available completions, instead of
wanting a partially completed word.  So this case might need special
handling.

2. Minibuffer content rewriting.

The current minibuffer content rewriting facility in
completion--do-completion seems not for partial completion.  Partial
completion normally returns 0 as completion point, e.g. ("org--" . 0),
so a rewriting may look like this (`|' means the position of cursor):

  M-x org-|
  (typing SPC)
  M-x |org--
  (typing SPC again, a surprising result from an attempt of completing
   an empty string):
  M-x -org-|-

Something is apparently wrong.

Regards,
Guanpeng Xu
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