Windows: Use a relative default template_dir and ETC_GITCONFIG

With this definition the templates and system config file will be found
irrespective of the installation location.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

Makefile

@@ -739,6 +739,8 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	COMPAT_OBJS += compat/mingw.o compat/fnmatch.o compat/regex.o
 	EXTLIBS += -lws2_32
 	X = .exe
+	template_dir = ../share/git-core/templates/
+	ETC_GITCONFIG = ../etc/gitconfig
 endif
 ifneq (,$(findstring arm,$(uname_M)))
 	ARM_SHA1 = YesPlease