perf completion: Introduce a layer of indirection

Define the variables cur, words, cword, and prev outside the main
completion function so that we have a chance to override it when we
introduce zsh support.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1384704807-15779-2-git-send-email-artagnon@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>