completion: complete remote names too

"git remote update" accepts both groups and single remotes.

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Reviewed-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2631,7 +2631,7 @@ _git_remote ()
 		__gitcomp_builtin remote_update
 		;;
 	update,*)
-		__gitcomp "$(__git_get_config_variables "remotes")"
+		__gitcomp "$(__git_remotes) $(__git_get_config_variables "remotes")"
 		;;
 	set-url,--*)
 		__gitcomp_builtin remote_set-url