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

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>