completion: start moving to the new zsh completion

completion: start moving to the new zsh completion

Zsh's bash completion emulation is buggy, not properly maintained, and
we have some workarounds in place for different bugs that appeared in
various versions.

Since I'm the only one that has worked on that code lately[1], it might make
snese to use the code I wrote specifically for git.

The advantages are:

 1) Less workarounds

   * No need to hack __get_comp_words_by_ref
   * No need to hack IFS or words

 2) Improved features

   * 'git show master' now properly adds a space at the end (IFS bug)
   * 'git checkout --conflict=' now properly returns the sub-items
     (missing feature)

 3) Consolidated code

   * It's all now in a single chunk, and it's basically the same as
     git-completion.zsh

Since there's some interest in moving the zsh-specific code out of this
script, lets go ahead and warn the users that they should be using
git-completion.zsh.

[1] http://zsh.git.sourceforge.net/git/gitweb.cgi?p=zsh/zsh;a=history;f=Completion/bashcompinit

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>