mailmap: consolidate multiple addresses into one

mailmap: consolidate multiple addresses into one

Merges contributions made from three different addresses:

- win@wincent.com (old address, initial contributions in 2007–2009)
- greg@hurrell.net (personal address matching full name, so this one is
  the "forever" address; contributions made starting in 2018)
- greg.hurrell@datadoghq.com (current work address, used for recent
  contributions)

Signed-off-by: Greg Hurrell <greg.hurrell@datadoghq.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>