git-p4: add file streaming progress in verbose mode

git-p4: add file streaming progress in verbose mode

If a file is streamed from P4 to Git then the verbose mode prints
continuously the progress as percentage like this:
//depot/file.bin 20% (10 MB)

Upon completion the progress is overwritten with depot source, local
file and size like this:
//depot/file.bin --> local/file.bin (10 MB)

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>