xread: retry after poll on EAGAIN/EWOULDBLOCK

We should continue to loop after EAGAIN/EWOULDBLOCK as the
intent of xread is to try until there is available data,
EOF, or an unrecoverable error.

Fixes: 1079c4be0b ("xread: poll on non blocking fds")

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -252,6 +252,7 @@ ssize_t xread(int fd, void *buf, size_t len)
 				 * call to read(2).
 				 */
 				poll(&pfd, 1, -1);
+				continue;
 			}
 		}
 		return nr;