help: use HTML as the default help format on Windows

help: use HTML as the default help format on Windows

When 'git help $cmd' is run without a format option (e.g. -w), the
'man' format is always used. On some platforms, however, manual page
viewers are not often available.

Introduce DEFAULT_HELP_FORMAT make variable in order to allow the
default format configurable at compile time, and set it to HTML when
compiling on Windows (but not Cygwin).

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Vincent van Ravesteijn <vfr@lyx.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>