(choose-completion): Use `buffer-substring-no-properties'

(choose-completion): Use `buffer-substring-no-properties'
instead of `buffer-substring'.
(completion-common-substring): Doc fix.
(completion-setup-function): Use minibuffer-completion-contents
instead of minibuffer-contents.  Don't set common-string-length
initially.  Remove special handling of partial-completion-mode.
Move computation of completion-base-size into one cond.  Call
completion-base-size-function in mainbuf.  In computation of
completion-base-size for file name completion don't move point to
the end of the minibuffer.  Move computation of common-string-length
into one cond.  Start putting faces only when common-string-length>=0.
Add condition to put completions-common-part when common-string-length>0.