gitweb: esc_html_hl_regions(): Don't create empty <span> elements

gitweb: esc_html_hl_regions(): Don't create empty <span> elements

If $end is equal to or less than $begin, esc_html_hl_regions()
generates an empty <span> element.  It normally shouldn't be visible in
the web browser, but it doesn't look good when looking at page source.
It also minimally increases generated page size for no special reason.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Acked-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>