Make read_in_full() and write_in_full() consistent with xread() and xwrite()

xread() and xwrite() return ssize_t values as their native POSIX
counterparts read(2) and write(2).

To be consistent, read_in_full() and write_in_full() should also return
ssize_t values.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -91,7 +91,8 @@ int validate_headref(const char *path)
 	struct stat st;
 	char *buf, buffer[256];
 	unsigned char sha1[20];
-	int len, fd;
+	int fd;
+	ssize_t len;
 
 	if (lstat(path, &st) < 0)
 		return -1;