parse-options: simplify usage_padding()

parse-options: simplify usage_padding()

c512643e67 (short help: allow a gap smaller than USAGE_GAP, 2023-07-18)
effectively did away with the two-space gap between options and their
description; one space is enough now.  Incorporate USAGE_GAP into
USAGE_OPTS_WIDTH, merge the two cases with enough space on the line and
incorporate the newline into the format for the remaining case.  The
output remains the same.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>