perf completion: Factor out call to __ltrim_colon_completions

perf completion: Factor out call to __ltrim_colon_completions

In our sole callsite, __ltrim_colon_completions is called after
__perfcomp, to modify the COMPREPLY set by the invocation.

This is problematic, because in the zsh equivalent (using compset/
compadd), we'll have to generate completions in one-shot.

So factor out this entire callsite into a special override'able
__perfcomp_colon function; we will override it when introducing zsh
support.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1384704807-15779-4-git-send-email-artagnon@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>