completion: simplify __git_remotes()

We spend great effort in __git_remotes() to get the list of remotes as
we list the contents of the .git/remotes directory and processes the
output of a 'git config' query.  And all that to get the same result
as as a simple 'git remote' would give us directly.

So spare all this complexity and just run 'git remote'.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
completion: remove redundant __git_compute_all_commands() call

In the code path to get a list of 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()
    __git_list_porcelain_commands()
      __git_compute_all_commands()

Either one of the two calls is redundant, 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>