avoid 31-bit truncation in write_loose_object

The size of the content we are adding may be larger than
2.1G (i.e., "git add gigantic-file"). Most of the code-path
to do so uses size_t or unsigned long to record the size,
but write_loose_object uses a signed int.

On platforms where "int" is 32-bits (which includes x86_64
Linux platforms), we end up passing malloc a negative size.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2280,7 +2280,8 @@ static int create_tmpfile(char *buffer, size_t bufsiz, const char *filename)
 static int write_loose_object(const unsigned char *sha1, char *hdr, int hdrlen,
 			      void *buf, unsigned long len, time_t mtime)
 {
-	int fd, size, ret;
+	int fd, ret;
+	size_t size;
 	unsigned char *compressed;
 	z_stream stream;
 	char *filename;