perf completion: Strip dependency on bash-completion

The bash-completion package defines the _get_comp_words_by_ref function.
There is no need to depend on it, as we can reimplement it like git.git
has.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Link: http://lkml.kernel.org/r/1372941691-14684-7-git-send-email-artagnon@gmail.com


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