bug#24888: pcomplete/cd returns "../" before "./" and does not include empty completion in results of pcomplete-at-point

[Clément Pit--Claudel]

pcomplete's ordering of completion makes it inconvenient to input directory 
names to 'cd' to when using company.  Example scenario:

* Start a shell (M-x shell)
* Enable company-mode
* Start typing `cd /home/clement/`

This invokes (assuming a low enough completion delay) pcomplete-at-point (the 
call goes through company-capf, which relies on completion-at-point-functions, 
which contains comint-dynamic-completion, which includes pcomplete-at-point).  
The results of this pcomplete invocation include "../" and "./", followed by 
name-ordered file names.  There are two problems:

* "../" comes first, which causes the default completion to be 
"/home/clement/../".
* pcomplete's list doesn't include "/home/clement/" itself.

Fixing the second problem would fix the first one; if pcomplete included "" in 
its results in addition to "./" and "../", and ranked it first, there would be 
no problem (company would display "/home/clement/" first, just like 
company-files does)

Cheers,
Clément.

In GNU Emacs 26.0.50.14 (x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2016-10-29 built on clem-w50-mint
Repository revision: fae796fe2695b419aa9277d243633bae400a147a
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:     Linux Mint 18 Sarah

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