completion: use 'sort -u' to deduplicate config variable names

The completion script runs the classic '| sort | uniq' pipeline to deduplicate the output of 'git help --config-for-completion'. 'sort -u' does the same, but uses one less external process and pipeline stage. Not a bit win, as it's only run once as the list of supported configuration variables is initialized, but at least it sets a better example for others to follow. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>