convert: The native line-ending is \r\n on MinGW

If you try this:

 1. Install Git for Windows (from the msysgit project)

 2. Put

	[core]
		autocrlf = false
		eol = native

    in your .gitconfig.

 3. Clone a project with

	*.txt text

    in its .gitattributes.

Then with current git, any text files checked out have LF line
endings, instead of the expected CRLF.

Cc: Johannes Schindelin <johannes.schindelin@gmx.de>
Cc: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -507,6 +507,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 		compat/win32/dirent.o
 	EXTLIBS += -lws2_32
 	PTHREAD_LIBS =
+	NATIVE_CRLF = YesPlease
 	X = .exe
 	SPARSE_FLAGS = -Wno-one-bit-signed-bitfield
 ifneq (,$(wildcard ../THIS_IS_MSYSGIT))