git-completion.bash: silence "not a valid object" errors

Trying to complete the command

  git show master:./file

would cause a "Not a valid object name" error to be output on standard
error. Silence the error so it won't appear on the command line.

Signed-off-by: Dylan Smith <dylan.ah.smith@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -676,7 +676,7 @@ __git_complete_revlist_file ()
 		*)   pfx="$ref:$pfx" ;;
 		esac
 
-		__gitcomp_nl "$(git --git-dir="$(__gitdir)" ls-tree "$ls" \
+		__gitcomp_nl "$(git --git-dir="$(__gitdir)" ls-tree "$ls" 2>/dev/null \
 				| sed '/^100... blob /{
 				           s,^.*	,,
 				           s,$, ,