Show upload progress when uploading osmChange

[avarab@…]

When JOSM uploads osmChange data it doesn't show a progress bar. This could be implemented by counting the number of bytes of the osmChange request that have been successfully sent over
the wire.

I looked at the relevant code in JOSM once but I couldn't find a way
to do it (but I'm not familiar with Java). In Perl with
HTTP::Request::Common you can do e.g.:

    $HTTP::Request::Common::DYNAMIC_FILE_UPLOAD = 1
    my $req = HTTP::Request::Common->new( ... );
    $req->content( sub {
        my $chunk = &$gen();
        my $length = length $chunk;

        warn "I'm now uploading a chunk of length $length, of a total
of $total bytes";

        return $chunk;
     } );
     my $result = LWP::UserAgent->request( $req );

(See this code for a practical example of this that I wrote:
​http://cpansearch.perl.org/src/CPB/Flickr-Upload-1.32/Upload.pm)

I presume Java has some library to do this as well but I couldn't find it.