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 e15098a (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>

Author: newren

contrib/completion/git-completion.bash

@@ -3084,7 +3084,7 @@ __gitcomp_directories ()
 			COMPREPLY+=("$c/")
 			_found=1
 		fi
-	done < <(git ls-tree -z -d --name-only HEAD $_tmp_dir)
+	done < <(__git ls-tree -z -d --name-only HEAD $_tmp_dir)
 
 	if [[ $_found == 0 ]] && [[ "$cur" =~ /$ ]]; then
 		# No possible further completions any deeper, so assume we're at