completion: squelch stray errors in sparse-checkout completion

completion: squelch stray errors in sparse-checkout completion

If, in the root of a project, one types

    git sparse-checkout set --cone ../<TAB>

then an error message of the form

    fatal: ../: '../' is outside repository at '/home/newren/floss/git'

is written to stderr, which munges the users view of their own command.
Squelch such messages by using the __git() wrapper, designed for this
purpose; see commit e15098a314 (completion: consolidate silencing errors
from git commands, 2017-02-03) for more on the wrapper.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>