bug#37189: 25.4.1: vc-hg-ignore implementation is missing

[Wolfgang Scherer]

Hi Eli,

coming to think of it, I realized that there probably is no maintainer for the 
vc ignore feature. I.e., there is no use in explaining the design with many 
words, since it will not be implemented anyway.

So I went ahead and implemented it myself. The CVS, GIt and Hg subsystems are 
tested and work as expected. The code and a short description is available for 
review at 
http://sw-amt.ws/README-emacs-vc-ignore-feature.html#vc-ignore-parameter-extension

If you agree that this is a good way to proceed, I will integrate it.

Wolfgang