completion: remove redundant __git_compute_all_commands() call

completion: remove redundant __git_compute_all_commands() call

During lazy-initialization of the lists of all commands and porcelain
commands the function __git_compute_all_commands() is called twice.  The
relevant part of the call sequence looks like this:

  __git_compute_porcelain_commands()
     __git_compute_all_commands()
        <finds list of all commands uninitialized>
        __git_list_all_commands()
        <initializes list of all commands>
     __git_list_porcelain_commands()
        __git_compute_all_commands()
           <finds list of all commands already initialized, does nothing>
        <filters porcelains from list of all commands>

Either one of the two calls could be removed and the initialization of
both command lists would still work as a whole, but let's remove the call
from __git_compute_porcelain_commands(), because this way
__git_list_porcelain_commands() will keep working in itself.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>