Tag GIT_VERSION when Git is built with MSVC

This may help us debug issues on Windows, as we now can build Git
natively on Windows with both MinGW and MSVC.

Signed-off-by: Marius Storm-Olsen <mstormo@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -877,6 +877,7 @@ ifneq (,$(findstring CYGWIN,$(uname_S)))
 	UNRELIABLE_FSTAT = UnfortunatelyYes
 endif
 ifdef MSVC
+	GIT_VERSION := $(GIT_VERSION).MSVC
 	pathsep = ;
 	NO_PREAD = YesPlease
 	NO_OPENSSL = YesPlease