bug#1512: 23.0.60; SPC, TAB during completion do not do word completion, prefixcompletion

[Drew Adams]

To be more exact, it seems that even what you call `basic' completion is
different from past Emacs behavior. 

It is apparently what you call `emacs21' that is closest to traditional Emacs
behavior. This comment in minibuffer.el states that
`completion-emacs22-try-completion' is a misnomer:

 ;; Merge a trailing / in completion with a / after point.
 ;; We used to only do it for word completion, but it seems to make
 ;; sense for all completions.
 ;; Actually, claiming this feature was part of Emacs-22 completion
 ;; is pushing it a bit: it was only done in minibuffer-completion-word,
 ;; which was (by default) not bound during file completion, where such
 ;; slashes are most likely to occur.

In any case, any of the following are probably close enough to the traditional
behavior, and any of them would be acceptable as the default value of
`completion-styles': 

 (basic)   ; Emacs 23 word and prefix completion
 (emacs22) ; Emacs 22 word and prefix completion
 (emacs21) ; Emacs 21 word and prefix completion

They all complete a word with SPC and complete a prefix with TAB, signalling [No
match] whenever the word or prefix completion fails.

What's not appropriate is, as is currently done, adding partial-completion into
the mix as the default value of `completion-styles':

 (basic partial-completion)

And it would make sense to change the name of `basic' to `emacs23' - it is the
Emacs 23 version of word and prefix completion.