Attempt to fully read into buffer during file compare (#1495)

The kernel (or GVFS backend) is allowed to return from the read call when it hasn't fully filled up the buffer, even if we haven't reached EOF yet. This is usually not going to happen for a local file, but can easily occur for remote filesystems or other protocols such as using MTP with a phone or other device.

The solution to this is to instead use g_input_stream_read_all(), which will block until the buffer has been filled, it's reached EOF, or an error occurs.

Closes #1495 (closed)