MinGW: Teach Makefile to detect msysgit and apply specific settings

MinGW: Teach Makefile to detect msysgit and apply specific settings

This commit changes handling of the msysgit specific settings, so
that they can be applied to official git.git.  Some msysgit
settings differ from the standard MinGW settings.  We move them
into an ifndef block that is only evaluated if a file
THIS_IS_MSYSGIT is present in the parent directory, which is the
case for an msysgit working environment.  The tag file is unlikely
to be present accidentally.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>